Columbia  ^nibersJitp  I  o    .^^ 

in  tfje  €itv  of  i?etD  gorfe      \  *  ^ 


COLLEGE  OF  PHYSICIANS 
AND  SURGEONS 


Reference  Library 

Given  by 


uK^t^J^^^ZJ  /q^ 


NUTRITIONAL 


PHYSIOLOGY 


BY 


PERCY    GOLDTHWAIT    STILES 

ASSISTANT      PROFESSOR      OF     PHYSIOLOGY    IN      SIMMONS 
COLLEGE  ;    INSTRUCTOR  IN  PHYSIOLOGY  AND    PERSONAL 
HYGIENE    IN  THE    MASSACHUSETTS    INSTITUTE  OF  TECH- 
NOLOGY, BOSTON 


ILLUSTRATED 


PHILADELPHIA  AND  LONDON 

W.  B.  SAUNDERS  COMPANY 

1912 


1 — "!'  ■'■'  '111         nil  nil  Mil 


ZP^SSZZ 


TFlulu^ 


Copyright,  1912,  by  W.  B.  Saunders  Company 


PRINTED    IN    AMERICA 

PRESS    OF 

W.     B.     SAUNDERS     COMPANY 

PHILADELPHIA 


or 


TO 

©rabam  Xush 

WHOSE  DELICATE  KINDNESS  NO  LESS  THAN  HIS 

SCHOLARLY    POWER    MADE    MEMORABLE    THE 

YEAR  OF  OUR  ASSOCIATION 


PREFACE 


The  making  of  this  book  has  been  a  study  in  elimination. 
It  is,  therefore,  necessary  at  the  outset  to  indicate  the 
Hmits  of  its  scope.  It  is  intended  that  it  shall  be  used 
with  other  books.  Supplementary  reading  upon  general 
biology,  human  anatomy,  food  chemistry,  and  dietetics 
is  greatly  to  be  desired.  In  the  field  of  physiology  itself 
many  fascinating  topics  are  entirely  ignored  and  others 
treated  in  bare  outline,  with  the  purpose  of  subordinating 
all  else  to  the  subject  of  nutrition.  Chemical  formulae 
have  been  excluded  from  the  text  and  used  but  sparingly 
in  the  notes. 

A  certain  preliminary  knowledge  of  elementary  science 
is  assumed.  The  key-word  of  the  following  discussion  is 
''  energy."  The  success  of  the  reader  in  gaining  clear  con- 
ceptions of  what  is  presented  will  depend  upon  his  famil- 
iarity with  the  meaning  of  that  term.  It  is  essential  that 
he  shall  understand  that  energy  is  latent  or  potential  in 
those  chemical  compounds  which  are  susceptible  of  oxida- 
tion. He  must  have  learned  to  recognize  the  possibility 
of  its  unending  transformation.  The  more  readily  he 
thinks  in  terms  of  molecules,  the  more  profitably  he  can 
read  these  chapters. 

Miss  Ruth  Bryant,  Instructor  in  Biology  in  Simmons 
College,  has  borne  a  part  in  the  work,  which  is  to  be  de- 
scribed as  collaboration  rather  than  assistance. 

P.  G.  S. 

Boston,  Mass., 
August,  1912. 

5 


TABLE  OF  CONTENTS 


CHAPTER  I  PAGE 

Introduction 11 

CHAPTER  II 

The  Energy  Relations  of  Plants  and  Animals 20 

CHAPTER  III 
The  Nature  and  the  Means  of  Digestion 28 

CHAPTER  IV 
The  Work  of  Muscles  and  Glands 36 

CHAPTER  V 
Reflex  Action 46 

CHAPTER  VI 
The  Alimentary  Canal 54 

CHAPTER  VII 
The  Mouth — Swallowing,  Salivary  Digestion 63 

CHAPTER  VIII 
The  Movements  of  the  Stomach 70 

CHAPTER  IX 

Gastric  Secretion  and  Digestion 78 

CHAPTER  X 

The  Small  Intestine:   Its  Movements,   Secretions,   and 

Digestive  Processes 88 

7 


8  '  TABLE    OF   CONTENTS 

CHAPTER  XI 

PAGE 

The  Large  Intestine 98 

CHAPTER  XII 
The  Blood 105 

CHAPTER  XIII 
The  Circulation 116 

CHAPTER  XIV 
The  Absorption  of  Food-stuffs 128 

CHAPTER  XV 
The  Metabolism  of  Fats  and  Carbohydrates 135 

CHAPTER  XVI 
Nitrogenous  Metabolism 147 

CHAPTER  XVII 
The  Removal  of  the  End-products  of  Metabolism 160 

CHAPTER  XVIII 
The  Estimation  of  Metabolism 169 

CHAPTER  XIX 
The  Energy  of  the  Metabolism 176 

CHAPTER  XX 
The  Factors  which  Modify  Metabolism 187 

CHAPTER  XXI 
The  Maintenance  of  the  Body  Temperature 196 

CHAPTER  XXII 
The  Hygiene  of  Nutrition 204 


TABLE   OF  CONTENTS  9 

CHAPTER  XXIII 

PAGE 

The  Hygiene  of  Nutrition  (Continued) 217 

Water;  Meat;  Sugar. 

CHAPTER  XXIV 
Alcohol 227 

CHAPTER  XXV 
Internal  Secretion 238 

CHAPTER  XXVI 
The  Nervous  System 245 

CHAPTER  XXVII 
The  Nervous  System — Its  Higher  Work 255 

INDEX 265 


NUTRITIONAL  PHYSIOLOGY 


CHAPTER  I 
INTRODUCTION 


The  Modern  Emphasis  in  Biology. — Living  things  are 
transformers  of  matter  and  energy.  When  we  say  trans- 
formers rather  than  generators  we  indicate  the  modern 
as  contrasted  with  the  old-time  view.  When  we  say  that 
physiology  is  the  physics  and  chemistry  of  living  matter 
we  suggest  the  same  significant  tendency  to  bring  living 
and  lifeless  matter  into  direct  comparison  and  to  recog- 
nize the  same  laws  as  operating  in  both.  The  teaching 
that  the  same  laws  do  hold  sway  in  the  living  and  the  non- 
living is  covered  by  the  term  "  mechanism  ";  the  earlier 
view  that  living  things  are  not  fairly  to  be  compared  with 
lifeless,  and  are  to  some  extent  exempt  from  physical 
principles  and  limitations,  is  expressed  by  the  word 
"  vitalism."  We  have  every  reason  to  believe  that  the 
principle  of  the  conservation  of  energy  holds  as  rigidly 
for  the  plant  or  the  animal  as  for  the  clock  or  the  loco- 
motive. This  is  perhaps  the  most  important  generaliza- 
tion of  nineteenth  century  physiology. 

But  while  scientific  workers  are  now  seeking  to  analyze 
the  reactions  of  organisms  in  accordance  with  the  data 
furnished  by  chemists  and  physicists  whose  work  has 
been  with  materials  not  living,  it  is  probable  that  the  diffi- 
culties of  their  problems  are  better  appreciated  than  was 
the  case  a  few  years  ago.  Living  matter  is  found  to  be 
more  complex  in  structure  and  more  varied  in  response 

11 


12 


NUTRITIONAL   PHYSIOLOGY 


than  had  been  supposed.  Physiologists  are  bound  to  be 
modest  in  their  claims  for  progress.  They  are  ignorant 
of  many  factors  at  work  in  even  the  simplest  forms  of 
plant  and  animal  life.  And  the  mystery  of  consciousness 
with  its  relation  to  nervous  systems  seems  ever  to  defy 
approach. 

Free-living  Cells. — About  seventy  years  ago,  at  a  time 
when  investigators  were  profiting  by  important  im- 
provements in  microscopes,  it  was  found  that  the  larger 


c 


Fig.  1. — Four  types  of  free-living  animal  cells:  A  is  the  ameba, 
distinguished  for  its  changeable  form;  B,  the  euglena,  shows  the 
peculiar  feature  known  as  a  flagellum,  a  writhing  filament,  which 
is  its  means  of  locomotion;  C  is  the  paramcecium,  or  "  slipper  ani- 
malcule," which  has  a  ciliated  surface;  D  is  the  interesting  form 
known  as  the  stentor. 


plants  and  animals  are  made  up  of  structural  units  as- 
sembled in  vast  numbers.  These  units  are  generally 
much  too  small  to  be  seen  without  magnification.  They 
are  called  cells,  a  term  which  is  not  especially  appropriate, 
but  not  likely  to  be  abandoned.  Many  microscopic 
forms,  such  as  the  swarming  Infusoria  of  pools  and  ditches, 
are  cells  leading  an  independent  existence.  It  will  be 
helpful  to  consider  what  are  the  characteristic  activities 
of  such  cells.  They  are  for  the  most  part  equally  charac- 
teristic of  the  higher  forms. 


INTRODUCTION  13 

The  free-living  animal  cell  takes  something  from  its  en- 
vironment and  returns  something  to  it.  It  takes  into  it- 
self a  variety  of  organic  substances  together  with  small 
quantities  of  mineral  salts.  These  constitute  its  food. 
It  receives  also  a  supply  of  oxygen.  This  is  not  ordinarily 
reckoned  as  a  food  and  for  a  good  reason.  The  term  food 
is  best  restricted  to  material  which  can  serve  constructive 
purposes  or  at  least  be  stored  in  the  cell.  The  function 
of  oxygen  is  not  to  promote  constructive  processes,  but  to 
release  energy,  a  process  of  decomposition  in  which  the 
stores  of  the  cell  are  sacrificed.  The  process  in  which 
oxygen  reacts  with  substances  within  the  cell,  giving  rise 
to  simple  oxidized  products  in  place  of  complex  material 
rich  in  potential  energy,  is  called  respiration.  (The  word 
is,  indeed,  frequently  used  as  a  synonym  for  breathing, 
but  we  shall  use  it  in  its  chemical  sense.)  Respiration 
is  often  compared  with  combustion,  and  while  the  two 
are  not  identical  in  all  their  stages,  the  fact  remains  that 
the  initial  and  the  final  conditions  are  essentially  the  same 
for  both.  The  release  of  energy  is  generally  just  as  great 
in  the  physiologic  change  as  in  the  actual  burning  of  like 
quantities  of  the  cell  constituents. 

The  free-living  animal  cell  is  thus  an  accumulator  of 
fuel  and  a  furnace  in  which  it  is  burnt.  But  this  is  a  very 
imperfect  comparison,  for  it  has  in  addition  the  property 
of  self-repair,  and  under  favorable  circumstances  capacity 
for  growth  and  reproduction.  Cells  multiply  by  cleaving 
into  two  similar  parts,  and  the  tendency  to  do  this  after 
a  certain  increase  in  size  usually  limits  very  definitely  the 
dimensions  to  which  a  single  cell  may  attain.  When 
growth  is  taking  place  it  is  evident  that  not  all  the  food 
is  serving  as  fuel;  a  certain  portion  is  becoming  incor- 
porated with  the  more  permanent  substance  of  the  cell 
and  is  so  changed  as  to  become  entirely  typical  protoplasm. 
The  process  through  which  food  becomes  an  integral  part 
of  the  cell  is  called  assimilation.  The  word  emphasizes 
through  its  root-meaning  the  attainment  of  likeness  to 
the  material  of  the  cell  and  indirectly  implies  that  the  food 


14  NUTRITIONAL   PHYSIOLOGY 

was  originally  foreign  in  its  nature.  We  use  the  expression 
in  much  the  same  sense  when  we  speak  of  the  assimilation 
of  immigrant  peoples.  The  word  nutrition  is  used  in 
about  the  same  way  as  assimilation,  the  only  distinction 
being  that  we  speak  of  the  nutrition  of  cells  (or  cell- 
aggregates),  while  we  speak  of  the  assimilation  of  food, 
the  former  term  referring  to  the  structure  nourished  and 
the  latter  to  the  supplies  worked  over  for  the  purpose. 
The  word  digestion  is  best  restricted  to  the  preliminary 
stages  of  the  assimilation  process;  its  application  will  be 
defined  later. 

Respiration  has  been  said  to  be  a  process  in  course  of 
which  compounds  are  decomposed  that  their  potential 
energy  may  be  made  available.  The  greater  part  of  the 
released  energy  appears  as  heat.  A  smaller  part  mani- 
fests itself  in  movements  through  which  resistances  are 
overcome.  The  facts  in  regard  to  the  production  of 
energy  are  naturally  better  known  for  the  larger  animals 
than  for  the  free-living  cells,  but  ''  the  whole  is  equal  to 
the  sum  of  its  parts."  The  energy  from  respiration  may 
in  exceptional  cases  become  kinetic,  in  the  form  of  light 
or  electric  discharge  (firefly,  electric  eel).  The  energy 
which  shows  itself  in  movement  is  of  particular  interest 
to  us.  Motion,  when  exhibited  by  animal  cells,  is  almost 
always  the  expression  of  contraction,  the  word  being  used 
in  a  physiologic  sense.  So  used,  contraction  does  not  mean 
diminution  of  volume,  but  does  mean  diminution  of  sur- 
face and  active  shortening  in  one  or  more  dimensions. 
Although  an  increase  in  other  dimensions  attends  such 
changes  of  form,  we  do  not  talk  of  the  "  expansion  "  of 
cells.  It  is  the  contraction  which  is  the  positive  and 
forcible  element  in  the  movement.  When  this  is  said  we 
intentionally  leave  out  of  account  some  types  of  movement 
occurring  commonly  in  the  plant  world,  in  course  of  which 
the  cells  actually  change  their  volume  through  gain  or  loss 
of  water.  Among  free-living  cells  the  type  of  move- 
ment may  be  "  ameboid,"  that  is,  a  flowing  of  the  cell 
contents  to  conform  to  an  ever-changing  outline.     Con- 


INTRODUCTION  15 

tractile  power  may  be  limited  in  other  cases  to  parts  of 
cells,  as  in  those  forms  which  swim  by  the  lashing  of 
slender  projections  known  as  flagella,  or  by  the  waving 
of  an  animated  nap  or  pile  upon  their  surfaces,  the  cilia, 
of  which  more  will  be  said.  In  all  cases  of  energetic 
movement  we  feel  justified  in  assuming  that  the  source 
of  the  power  is  in  destructive  chemical  reactions,  and 
that  a  draft  is  being  made  upon  the  fuel  stores  of  the 
cells. 

The  Association  of  Cells. — When  many  cells  are  massed, 
as  in  the  body  of  a  worm,  the  situation  of  the  single  unit 
differs  significantly  from  that  of  the  cell  leading  an  inde- 
pendent existence.  First  of  all,  its  environment  is  made 
for  it  to  a  great  extent  by  other  cells.  A  very  small 
minority  are  in  direct  contact  with  the  outside  world; 
the  great  majority  are  submerged  among  their  fellows. 
The  typical  cell  is,  therefore,  shut  in  from  food  supplies 
of  the  casual  sort  on  which  the  free-living  cell  depends. 
It  is  remote  from  the  oxygen  of  the  surrounding  air  or 
water.  A  cell  so  situated  would  perish  were  it  not  for  one 
of  the  most  striking  features  of  the  larger  organisms,  a 
moving  liquid  mediuniy  which  bathes  the  cells  and  acts 
as  a  common  carrier.  This  fluid  supplies  food  and  oxygen 
and  removes  wastes. 

The  cells  composing  the  body  of  any  animal  are  of  a 
common  descent,  but  they  have  taken  on  widely  different 
characters  and  have  become  adapted  to  particular  func- 
tions. The  cell  which  is  in  itself  a  complete  living  thing 
must  perform  all  the  essential  activities  for  itself — the 
preparation  of  crude  food,  locomotion,  etc.  In  the 
multicellular  animal  the  individual  cells  have  come  to  be 
far  more  restricted  in  their  powers.  Many  have  become 
passive  structures  serving  only  for  mechanical  support  or 
surface  protection.  Such  cells  may  or  may  not  be  living. 
Others,  while  clearly  alive,  have  ceased  to  perform  cer- 
tain functions.  With  limited  exceptions  movement  is 
exhibited  only  by  those  systematically  arranged  cells 
which  form  the  contractile  tissues.     Almost  all  the  cells 


16 


NUTRITIONAL    PHYSIOLOGY 


require  their  food  to  be  in  solution  and  of  a  few  standard 
forms.  In  other  words,  the  primitive  capacity  to  digest 
and  assimilate  every  kind  of  nutriment  has  been  lost,  but 
by  a  wonderful  co-operative  activity  the  internal  medium 
has  been  made  a  depot  of  those  particular  foods  which  can 
still  be  utilized. 


Fig.  2. — Drawings  like  the  above  are  almost  always  made  from 
tissues  which  have  been  prepared  and  colored  by  special  means  to 
make  clear,  minute  features:  a  Represents  an  ovum  or  egg-cell,  the 
typical  cell  may  be  assumed  to  tend  toward  this  spheric  form;  6  is  a 
cell  from  a  compact  tissue,  to  show  how  mutual  pressure  produces 
a  faceted  or  polyhedral  form;  c  is  a  contractile  element  such  as 
occurs  in  the  walls  of  the  alimentary  canal,  it  illustrates  an  elon- 
gated cell;  d  is  an  epithelial  or  lining  cell  of  the  order  found  on  the 
inner  surface  of  blood-vessels;  this  is  an  example  of  extreme  flatten- 
ing; e,  from  the  nervous  system,  exhibits  the  possibility  of  a  branch- 
ing development. 


As  an  animal  grows  larger  its  directly  exposed  surface 
becomes  smaller  in  proportion  to  its  weight.  The  trans- 
fers which  must  take  place  between  the  organism  and  the 
external  world  require  ample  surfaces,  and  they  are  secured 
by  infoldings  of  the  body  wall  at  different  places.  The 
lining  of  the  alimentary  tract  is  an  example  of  such  an 
infolding  and  provides  a  large  area  for  absorption.  Among 
the  higher  forms  the  lining  of  the  lungs  constitutes  a  vastly 


INTRODUCTION  17 

extended  surface  for  gaseous  exchange.  The  glands  are 
organs  in  which  are  concealed  great  surfaces  through 
which  products  of  cell  activity  find  an  outlet. 

The  specialization  which  groups  the  cells  of  the  animal 
body  in  a  number  of  classes  each  with  its  definite  work  to 
do  also  entails  the  dependence  of  each  class  upon  the 
others.  If  we  compare  the  life  of  a  savage  with  that  of  a 
civilized  man  we  shall  find  an  analogy  not  too  far  fetched 
to  be  helpful.  We  have  seen  that  the  free-living  cell  is 
self-sufficient,  and,  indeed,  its  chances  of  survival  are  bet- 
ter when  there  are  but  few  of  its  own  kind  in  the  neighbor- 
hood. Such  organisms  are  competitive  rather  than  co- 
operative. Somewhat  in  the  same  way  the  solitary  savage 
may  be  capable  of  self-maintenance,  having  the  skill  to 
find  and  prepare  his  food  and  after  a  fashion  to  shelter 
and  clothe  himself.  The  civilized  man  is  accustomed  to 
look  to  many  other  men — and  women — to  supply  his  needs. 
Yet  if  the  man,  like  the  cell,  loses  something  of  ruggedness 
and  resourcefulness  through  becoming  a  member  of  a  com- 
plex society,  he  evidently  gains  time  and  opportunity  to 
concentrate  his  efforts  upon  a  special  pursuit.  It  is  very 
much  the  same  with  the  cell.  Our  analogy  fails,  as  such 
devices  are  prone  to  do,  when  we  consider  how  the  civil- 
ized man  may  become  a  hermit  or  a  Robinson  Crusoe, 
whereas  no  single  cell  detached  from  one  of  the  higher 
forms  can  exist  by  itself  for  any  length  of  time. 

Co-ordination. — We  have  emphasized  above  the  services 
rendered  to  the  organism  by  its  internal  medium.  The 
composition  of  the  circulating  fluid  is  influenced  by  the 
ever-varying  activities  of  all  the  organs  and  tissues. 
Accordingly,  a  contribution  made  to  this  medium  by 
any  group  of  cells  may  conceivably  modify  the  conduct 
of  any  other  group.  We  shall  meet  with  numerous 
instances  of  such  influence  exerted  through  the  chemical 
products  of  one  organ  upon  another  or  upon  the  system  as 
a  whole.  When  we  speak  of  an  animal  as  an  individual 
we  imply  that  the  parts  of  the  body  constantly  interact. 
It  is  this  interaction  which  makes  it  appropriate  to  regard 
2 


18  NUTRITIONAL   PHYSIOLOGY 

the  body  with  all  its  complexity  as  still  forming  a  unity 
rather  than  a  colony.  The  term  co-ordination  is  employed 
to  express  this  purposeful  working  together  of  all  parts 
for  the  advantage  of  the  whole.  The  means  of  co-ordina- 
tion may  be  chemical  as  just  now  suggested,  but  a  more 
conspicuous  agency  in  the  highly  developed  types  is  that 
of  the  nervous  system.  While  we  must  postpone  until 
a  later  time  any  detailed  description,  we  ought  to  indicate 
at  this  point  the  essential  contrast  between  the  two  modes 
of  co-ordination.  One  part  of  the  body  may  affect  an- 
other through  the  actual  despatch  of  material  to  it. 
When  the  influence  is  through  the  nervous  system  instead 
of  through  the  circulation  there  is  no  transfer  of  material. 
The  nerves  were  once  supposed  to  convey  a  fluid  of  strange 
properties,  but  the  fact  is  established  that  they  transmit  a 
form  of  energy  and  not  of  matter.  (The  temptation  to 
think  of  the  nerves  as  conductors  of  electric  currents  and 
to  compare  the  nervous  mechanism  with  a  telephone 
system  is  strong.  Guardedly  used,  the  comparison  is 
valuable,  but  it  is  a  symbolic  rather  than  a  literal  repre- 
sentation of  the  facts.  ''  Nerve  impulses,"  so  called,  are 
not  electric  currents  in  the  ordinary  sense.) 

In  a  great  nation  the  prosperity  of  any  section  must  de- 
pend to  a  large  degree  upon  the  commercial  exchanges 
taking  place  between  that  section  and  others.  Its  fac- 
tories may  depend  upon  the  mines  of  another  province 
for  coal  and  upon  still  another  for  raw  material.  Much  in 
the  same  way  a  single  organ  of  the  human  body  is  de- 
pendent upon  others.  A  muscle,  for  instance,  profits  by 
the  prepared  foods  or  fuels  forwarded  to  it  from  the  di- 
gestive tract  and  by  the  oxygen  borne  to  it  from  the  lungs. 
The  blood  in  this  case  is  serving  the  purpose  which  is  ef- 
fected by  trains  and  steamers  in  the  case  of  the  nation. 
Nor  do  we  find  lacking  in  our  illustration  an  analogy  for 
the  nervous  communication  between  parts  of  the  living 
body.  The  type  of  such  intercourse  is  furnished  by  the 
telegraphic  messages  which  pass  incessantly  from  place 
to  place.     News,  in  itself  immaterial,  may  affect  the  course 


INTRODUCTION 


19 


of  local  events  just  as  surely  and  much  more  quickly  than 
can  the  material  exports  of  another  region.  Reactions 
produced  through  the  nervous  system  are  correspondingly 
sharp  and  prompt  in  developing. 

Blood  and  Lymph. — In  the  bodies  of  the  higher  animals 
the  internal  medium  may  be  described  as  existing  in  two 
forms.  In  direct  contact  with  the  majority  of  the  cells 
there  is  a  comparatively  stagnant  fluid,  the  lymph. 
From  this  they  draw  their  needful  supphes  of  oxygen  and 


Fig.  3. — B  is  a  blood-vessel  of  the  smallest  size — a  capillary — 
with  walls  of  flattened  cells  like  that  in  Fig.  2,  d.  The  blood  flow- 
ing within  is  removed  from  direct  contact  with  the  cells  (C,  C),  but 
dissolved  substances  may  pass  from  one  to  the  other  through  the 
capillary  wall  and  the  medium  of  the  lymph  (L). 


food;  into  it  they  discharge  their  waste.  The  limited 
resources  of  the  lymph  at  a  given  point  would  be  quickly 
exhausted  were  it  not  that  the  blood  is  passing  close  by  in 
vessels  whose  delicate  walls  permit  the  passage  of  material 
in  both  directions.  The  blood  is  in  rapid  movement  and 
it  is  constantly  renewing  the  oxygen  of  the  l^inph  with 
fresh  portions  just  brought  from  the  lungs.  It  is  at  the 
same  time  receiving  from  the  lymph  the  accumulated 
waste. 


CHAPTER  II 

THE  ENERGY  RELATIONS   OF  PLANTS  AND 

ANIMALS 

A  CHEMICAL  reaction  can  usually  be  assigned  to  one  of 
two  classes.  Either  it  is  exothermic,  that  is,  attended  by 
the  evolution  of  heat,  or  it  is  endothermic,  in  which  case 
heat  must  be  supplied  to  cause  its  occurrence.  When 
heat  is  evolved  the  products  of  the  reaction  are  generally 
simpler  and  more  stable  than  the  original  material.  The 
most  important  reactions  of  this  class  are  the  oxidations. 
Heat,  or  other  forms  of  energy  applied  from  without,  may 
effect  the  synthesis  of  complex  substances  rich  in  fuel 
value  from  initial  material  comparatively  destitute  of 
potential  energy. 

Broadly  speaking,  the  constructive  chemical  processes 
in  nature  are  the  work  of  the  higher  plants.  Animals,  as 
well  as  those  forms  of  plant  life  which  lack  pigment,  carry 
on  for  the  most  part  reactions  of  a  destructive  character. 
This  makes  evident  the  dependence  of  all  other  forms  of 
living  matter  upon  the  pigmented  plants.  It  is  through 
the  agency  of  light-waves,  a  form  of  kinetic  energy,  that 
the  synthetic  reactions  resulting  in  the  formation  of  starch 
and  other  energetic  compounds  are  made  possible.  When 
light  is  excluded  from  the  green  plant  it  has  no  advantage 
over  the  animal,  but  pursues  a  similar  course  of  decomposi- 
tion. In  fact,  there  is  always  going  on  in  the  plant, 
even  when  its  constructive  activity  is  most  marked,  an 
undercurrent  of  an  opposite  trend.  Early  Avriters  com- 
monly exaggerated  the  supposed  contrast  between  the 
chemical  conduct  of  plants  and  that  of  animals.  They 
were  disposed  to  deny  that  an  animal  could  execute  any 

20 


ENERGY    RELATIONS    OF   PLANTS   AND    ANIMALS       21 

sjnithesis  whatever.  It  is  true  that  animal  cells  make  no 
useful  application  of  the  energy  showered  upon  them  by 
the  sun's  rays.  Nevertheless  they  do  carry  on  synthetic 
reactions,  although  to  a  limited  extent.  Since  much  energy 
is  released  within  such  cells  by  the  prevailing  oxidative 
changes  it  is  not  difficult  to  see  that  some  portion  of  it  may 
be  applied  to  promote  endothermic  reactions.  When  a 
hydraulic  ram  supplies  with  water  a  house  considerably 
above  the  level  of  the  stream  which  operates  the  device, 
we  understand  that  the  result  is  made  possible  because  a 
great  deal  of  water  falls  that  a  little  may  rise.  The  prin- 
ciple of  the  conservation  of  energy  is  not  violated  here, 
nor  is  it  when  animal  cells  erect  from  a  portion  of  their 
food  molecular  structures  more  complex  and  energetic 
than  anything  in  their  current  supply.  The  formation  of 
fat  from  sugar  is  a  case  in  point.  Weight  for  weight,  the 
fat  is  more  highly  endowed  with  potential  energy  than  is 
the  sugar,  but  we  must  take  into  account  the  fact  that  the 
quantity  of  sugar  entering  into  this  common  transforma- 
tion is  much  greater  than  the  quantity  of  fat  which  can 
be  produced.  The  energy  in  the  product  is  more  concen- 
trated, but  not  absolutely  larger  in  amount  than  it  was  in 
the  sugar. 

There  are  many  species  of  green  plants  which  are  uni- 
cellular, just  as  there  are  many  single-celled  animal  forms. 
It  is  suggestive  to  consider  the  reciprocal  relations  of  one 
such  plant  and  a  solitary  animal  cell  living  beside  it.  A 
constant  and  pressing  need  of  the  animal  is  oxygen.  Now 
oxygen  is  freely  produced  by  plants  when  they  avail 
themselves  of  the  energy  of  light  to  carry  on  constructive 
processes.  To  this  extent,  then,  the  proximity  of  the 
plant  to  the  animal  is  advantageous  to  the  latter.  This 
ceases  to  be  true  when  light  is  succeeded  by  darkness. 
The  animal  meanwhile  is  giving  off  oxidized  products,  of 
which  carbon  dioxid  is  the  most  abundant.  This,  to- 
gether with  water,  is  the  very  material  out  of  which  the 
plant  can  build  its  stores  of  starch  and  sugar.  The  out- 
put of  the  animal  includes  also  various  compounds  of 


22  NUTRITIONAL   PHYSIOLOGY 

nitrogen.  These,  as  well  as  the  carbon  dioxid,  may  be  of 
service  to  the  plant,  although  to  be  strictly  accurate  it  must 
be  added  that  they  require  some  modification,  usually 
effected  in  nature  through  the  action  of  bacteria.  In 
view  of  these  exchanges  one  is  tempted  to  the  hasty  con- 
clusion that  a  single-celled  green  plant  and  a  single- 
celled  animal  form  a  balanced  system  capable  of  con- 
tinued maintenance — in  short,  a  microcosm.  There  is, 
however,  a  fatal  obstacle  to  the  continuance  of  the  part- 
nership— the  animal's  need  of  organic  food  can  only  be 
satisfied  by  the  sacrifice  of  the  plant.  To  have  a  truly 
balanced  system  of  an  enduring  character  we  must  assume 
a  multiplication  of  cells  descended  from  the  original  uni- 
cellular plant,  providing  a  surplus  of  vegetable  tissue  for 
the  animal's  consumption. 

The  give  and  take  which  has  been  illustrated  for  single 
cells  is  proceeding  on  a  vast  scale  everywhere.  It  is 
hard  to  realize  that  the  great  harvests  which  support 
the  races  of  mankind  were  formed  for  the  most  part  from  a 
gas  present  only  very  sparingly  in  the  atmosphere,  from 
water,  and  from  the  mineral  salts  of  the  soil.  While  we 
rely  partly  upon  animal  food  (meat,  milk,  and  eggs), 
this  does  not  alter  the  fact  of  our  absolute  dependence 
upon  the  green  plants,  which  in  turn  owe  their  growth  to 
the  translated  energy  of  the  sun.  It  is  amusing  to  note 
the  apparent  travesty  upon  our  human  life  which  can  be 
read  into  the  contrasted  conduct  of  green  plants  and  of 
animals.  The  plants  are  conserving,  while  the  animals 
are  spendthrift.  Plants  create  and  distribute  wealth. 
They  seem  like  the  thrifty  and  industrious  members  of 
society  upon  whose  charity  others  less  competent  depend. 
One  is  reminded  irresistibly  of  the  parent  at  home  and 
the  son  at  college.  If  the  father  does  not  literally  subsist 
upon  a  diet  of  carbon  dioxid  and  water  that  the  son  may 
have  protein  and  alcohol,  the  approach  to  a  parallel  is  too 
close  for  complacent  attention. 

The  Body  and  the  Diet. — Turning  now  to  the  definite 
problems  of  human  nutrition,  let  us  consider  the  respective 


ENERGY    RELATIONS    OF   PLANTS    AND    ANIMALS       23 

make-up  of  the  body  and  the  diet.  There  must  evidently 
be  a  degree  of  similarity  between  them,  inasmuch  as  the 
one  has  been  built  from  the  other.  Similar  compounds 
are  met  with  in  both,  but,  as  we  shall  find,  in  quite  unlike 
proportions.  The  body  is  mainly  water.  Water  makes 
up  about  two-thirds  of  the  total  weight  and  forms  even  a 
larger  percentage  of  the  most  active  tissues.  No  material 
reduction  of  its  quantity  can  be  tolerated.  Even  after 
death  from  thirst  the  amount  is  surprisingly  little  dimin- 
ished. In  the  diet  also  water  occupies  the  first  place.  It 
is  Hkely  to  constitute  fully  five-sixths  of  the  daily  income. 
Its  most  obvious  services  are  in  connection  with  the 
absorption  of  food  in  solution  and  the  removal  of  dissolved 
wastes.  By  its  evaporation  from  the  skin  and  the  breath- 
ing passages  it  helps  to  keep  the  body  temperature  from 
rising  above  its  normal  level. 

Second  to  water  among  the  substances  which  compose 
the  body  we  find  the  group  of  bewilderingly  complex 
compounds  known  as  the  proteins.  A  protein  always 
yields  the  five  elements — carbon,  oxygen,  nitrogen, 
hydrogen,  and  sulphur^ — when  subjected  to  analysis. 
Some  members  of  the  group  contain  phosphorus  also. 
Merely  to  mention  these  constituent  elements  is  to  give 
no  proper  conception  of  the  intricate  manner  in  which 
they  must  evidently  be  combined.  To  appreciate  this 
we  need  to  consider  the  very  long  list  of  cleavage  products, 
in  themselves  rather  complex,  which  can  be  obtained  by 
the  decomposition  of  protein  from  a  single  source.  The 
physiologic  chemist  is  somewhat  in  the  position  of  a  person 

^  The  elements  occur  in  proteins  in  about  the  following  percentages: 
C  53,  O  22,  N  16,  H  7,  S  1  per  cent.  Phosphorus  when  present 
amounts  to  1  per  cent,  more  or  less.  It  is  quite  impossible  to  con- 
vey an  adequate  impression  of  the  complex  fashion  in  which  the  five 
or  six  elements  are  combined.  Many  years  ago  the  following  formula 
was  suggested  for  hemoglobin,  the  red  protein  of  the  blood,  which  is 
exceptional  in  containing  iron: 

It  is  not  seriously  maintained  that  these  large  numbers  are  precisely 
correct,  but  the  order  of  their  magnitude  is  probably  typical. 


24  NUTRITIONAL   PHYSIOLOGY 

who  sees  the  various  parts  of  a  watch — the  wheels,  springs, 
jewels,  etc. — lying  loose  upon  the  table  of  the  watch- 
maker. He  may  gain  a  fair  notion  of  the  intricacy  of  the 
watch,  though  he  may  be  very  far  from  knowing  how  the 
parts  were  related  in  the  time-piece.  We  do  well  to  use 
the  plural  number  in  speaking  of  the  proteins,  for  all  recent 
work  tends  to  emphasize  the  distinctive  molecular  pat- 
tern which  characterizes  each  form  and  makes  it  differ 
definitely  from  every  other.  "  There  is  one  flesh  of  men 
and  another  of  beasts  and  another  of  fishes  and  another  of 
birds."  This  is  excellent  and  altogether  modern  chemical 
biology.  The  presence  of  the  element  nitrogen  in  the 
proteins  distinguishes  them  sharply  from  the  other  promi- 
nent compounds  in  both  the  body  and  its  daily  income. 
Nitrogen  makes  up  about  16  per  cent,  of  protein,  and  the 
value  of  this  figure  in  calculations  will  be  apparent  later. 
It  has  been  said  that  the  proteins  are  second  only  to  water 
in  their  abundance  in  the  body.  This  is  not  true  of  the 
diet  unless  we  have  to  do  with  a  carnivorous  animal. 
Among  the  herbivora,  and  almost  always  among  men,  the 
second  place  in  the  list  of  supplies  is  occupied  by  the 
carbohydrates. 

Third  in  quantity  among  the  constituents  of  the  body 
we  find  in  most  individuals  the  mineral  compounds. 
These  would  not  make  so  large  a  proportion  if  it  were 
not  for  the  skeleton.  Bone  is  a  tissue  in  which  salts  of 
lime  are  abundantly  present.  But  in  all  the  other  tissues 
and  in  the  fluids  too  we  find  a  variety  of  salts,  and  it  is 
well  established  that  their  presence  is  not  accidental,  but 
a  matter  of  moment.  Sodium,  potassium,  calcium,  and 
magnesium  at  least,  perhaps  other  bases  also,  must  be 
kept  in  certain  balanced  relations  if  the  life  processes  are 
to  go  on.  The  acids  represented  are  chiefly  hydro- 
chloric, phosphoric,  and  carbonic.  Sodium  chlorid, 
the  one  salt  which  we  take  pains  to  add  to  our  food, 
is  the  one  most  abundant  in  blood  and  lymph.  Potas- 
sium rather  than  sodium  compounds  predominate  in  the 
cells. 


ENERGY    RELATIONS    OF    PLANTS    AND    ANIMALS       25 

Next  in  amount  to  the  salts  in  the  body  of  average  build, 
and  not  uncommonly  exceeding  them,  are  the  fats}  The 
word  ^'  fat  "  is  used  sometimes  in  a  chemical  and  some- 
times in  an  anatomic  sense.  In  the  first  case  it  denotes  a 
compound  of  carbon,  hydrogen,  and  oxygen,  having  a 
formula  of  a  certain  type.  In  the  other  usage  the  mean- 
ing is  "  adipose  tissue,"  a  form  of  connective  tissue  rich  in 
such  compounds.  Fats  have  familiar  physical  characters. 
They  are  not  soluble  in  water  or  to  any  great  extent  in  the 
fluids  of  the  body.  They  pass  from  solid  to  liquid  form 
at  moderate  temperatures;  the  fats  of  the  human  body 
are  regarded  as  in  a  fluid  state  when  under  the  influence  of 
its  warmth.  No  other  common  physiologic  compounds 
have  so  much  latent  energy  awaiting  release  by  oxidation. 
Fats  are  more  plentiful  in  apparently  lean  individuals  than 
might  be  judged.  A  considerable  store  of  adipose  tissue 
is  to  be  found  in  any  condition  short  of  imminent  starva- 
tion. 

It  has  been  said  above  that  carbohydrates  usually  have 
the  leading  place  among  the  solid  matters  of  the  diet. 
This  is  owing  to  the  large  proportion  of  vegetable  foods 
generally  consumed.  In  the  animal  body  the  occurrence 
of  carbohydrate  is  rather  scant,  and  it  is  one  of  the  chief 
problems  of  the  physiologist  to  account  for  the  daily  dis- 
appearance of  a  great  quantity  of  these  compounds  in  the 
economy  of  the  organism.  The  reader  may  already  foresee 
what  we  shall  later  explain  in  detail,  that  this  disappear- 
ance of  carbohydrate  is  due  in  part  to  the  fact  that  it  is 
the  fuel  most  constantly  called  upon  to  evolve  energy,  and 
in  part  to  the  ease  with  which  the  tissues  transform  to  fat 
a  surplus  of  these  substances.  Under  the  head  of  carbo- 
hydrates we  distinguish   the   starches   and   the   sugars. 

1  Fats  are  compounds  which  can  be  resolved  into  glycerin  and 
organic  acids.  Those  of  chief  interest  in  nutrition  are  the  glj^cerids 
of  palmitic,  stearic,  and  oleic  acids.  The  first  mentioned  has  a  com- 
position indicated  by  the  formula  C3H6(OOCH3iCi5)3.  The  others 
are  nearly  related.  The  three  common  fats  differ  in  their  melting- 
points  and  in  other  respects.  They  are  mingled  in  definite  propor- 
tions to  form  the  body  fat  of  each  animal  species. 


26  NUTRITIONAL   PHYSIOLOGY 

Starches^  are  of  high  molecular  complexity,  imperfectly 
soluble,  and  tasteless.  Sugars  are  cleavage  products  of 
starches  or,  if  they  occur  apart  from  previously  existing 
starches,  closely  resemble  such  cleavage  products.  They 
are  of  definite  and  moderate  molecular  weight,  they  are 
soluble,  crystallizable,  and  sweet.  They  contain  the  same 
elements  which  are  found  in  fats — carbon,  hydrogen,  and 
oxygen — but  the  percentage  composition  is  wholly  differ- 
ent and  the  structure  of  the  molecule  also.  While  the 
carbohydrates  are  energetic,  their  fuel  value  is  less  than 
half  that  of  fats. 

We  have  now  named  this  list  of  substances  as  uniting 
to  form  the  body — water,  proteins,  mineral  matter,  fats, 
and  carbohydrates — the  order  suggesting  their  relative 
abundance.  We  have  said  that  in  the  diet  water  also 
takes  the  first  place,  but  that  carbohydrates  ordinarily 
take  the  second.  Either  proteins  or  fats  may  stand  third 
among  the  constituents  of  the  diet.  Often  the  amounts 
of  the  two  are  found  to  be  about  equal.  A  possible  diet 
comprises  100  grams  of  protein,  100  grams  of  fat,  and  250 
grams  of  carbohydrate,  illustrating  this  equality.  The 
mineral  matter  in  the  ration  is  not  likely  to  exceed  20 
grams  a  day.  Both  the  body  and  its  income,  of  course, 
contain  in  small  amounts  substances  which  do  not  fall 
into  any  of  the  classes  named.  Such  miscellaneous 
organic  compounds  are  conveniently  grouped  as  extrac- 
tives. Many  of  them  are  nitrogenous  and  represent  dis- 
integration products  of  proteins.  Reference  to  their 
significance  will  be  made  from  time  to  time. 

At  the  very  outset  the  double  service  of  food  to   the 

^  The  three  elements  in  starch  are  present  in  proportions  repre- 
sented by  the  formula  CgHjoOg.  But  the  number  of  atoms  in  the 
molecule  is  not  correctly  indicated  by  this  formula;  it  requires  to 
be  multiplied  throughout  by  an  unknown,  but  considerable  number. 
Sugars  are  of  two  common  classes:  the  disaccharids,  with  the 
formula  C,2H220ii,  and  the  monosaccharids  or  hexoses,  with  the 
formula  CeHijOe-  Cane-sugar,  malt-sugar,  and  milk-sugar  are 
disaccharids.  The  hexoses  of  direct  interest  in  nutrition  are  glucose 
(also  called  dextrose),  fructose  (or  levulose),  and  galactose  (a  deriva- 
tive of  milk-sugar). 


ENERGY   RELATIONS   OF   PLANTS   AND  ANIMALS       27 

organism  was  indicated.  It  represents  both  building  mate- 
rial and  fuel.  If  we  liken  the  living  body  to  a  power-house, 
we  see  clearly  how  both  kinds  of  supplies  are  required. 
Coal  is  the  most  bulky  supply  of  the  power-plant  and  the 
one  on  which  its  operation  most  immediately  depends. 
But  there  must  also  be  new  parts  to  replace  those  which 
wear  out.  The  up-keep  of  the  building  calls  for  new 
wood-work,  for  paint,  etc.  A  certain  difficulty  is  encoun- 
tered in  the  attempt  to  show  parallel  conditions  in  the 
case  of  the  body  because  the  separation  of  the  two  func- 
tions is  here  much  less  sharp.  Protein  food,  on  the  whole, 
has  a  peculiar  title  to  be  regarded  as  building  material, 
but  it  is  also  an  entirely  available  fuel.  It  is  as  if  planks 
and  beams  designed  primarily  to  repair  the  structure  of  the 
power-house  were  fed  into  its  furnaces.  The  suggested 
comparison  must  not  be  pressed  too  far,  for  it  conveys 
an  impression  of  wanton  destructiveness  which  we  cannot 
assume  to  be  just  in  the  case  of  the  organism.  There  are 
some  minor  supplies  brought  into  the  power-house  which 
are  not  fuels  nor  precisely  materials  for  repair.  The  oil 
is  an  example.  Some  of  the  extractives  of  the  diet  occupy 
an  analogous  position,  being  neither  sources  of  energy  nor 
of  construction,  but  nevertheless  favorably  affecting  the 
course  of  events.  This  comes  near  to  our  conception  of  a 
drug  in  relation  to  the  processes  of  life. 


CHAPTER  III 
THE  NATURE  AND  THE  MEANS  OF  DIGESTION 

It  has  been  said  that  one  of  the  results  of  the  specializa- 
tion of  cells  is  the  loss  of  the  primitive  power  to  receive 
and  utilize  all  kinds  of  food.  The  blood  brings  to  the  tis- 
sues of  the  body  food  of  certain  standard  forms,  and  it  is 
only  these  which  can  be  used.  The  attempt  to  add  various 
soluble  foods  to  the  blood  by  direct  injection  into  the 
circulation  has  shown  that  in  most  cases  such  foods  are 
offered  in  vain  to  the  living  cells.  Milk  introduced  in 
this  way  is  not  a  practical  means  of  nutrition.  Cane-sugar 
added  in  measured  amounts  to  the  blood  is  excreted 
promptly  and  in  almost  undiminished  quantity  by  the 
kidneys.  Thus  it  becomes  clear  that  foods  introduced 
directly  into  the  blood  are  frequently  treated  like  waste 
products,  while  the  same  foods  after  transformation  in  the 
alimentary  canal  are  entirely  acceptable  to  the  body  cells. 
The  function  of  the  alimentary  canal  is  to  work  over  the 
many  foreign  forms  of  nutriment  into  a  few  forms  of  the 
native  type.  From  day  to  day  the  diet  may  be  of  quite 
variable  character,  but  its  variations  hardly  show  them- 
selves in  the  composition  of  the  blood. 

The  term  digestion  is  usually  applied  to  those  changes  in 
the  food-stuffs  which  precede  absorption.  To  cover  sub- 
sequent changes  we  use  the  word  metabolism. 

One  of  the  more  evident  characteristics  of  digestion  is 
that  it  is  a  refining  process.  It  effects  a  separation  of  the 
useful  and  the  useless  portions  of  the  ration.  This  is 
a  very  conspicuous  fact  with  the  herbivora,  whose  food 
contains  much  woody  material  from  which  the  available 
nutriment  must  be  laboriously  extracted.  With  man- 
kind, especially  under  modern  conditions,  a  great  part  of 

28 


THE    NATURE    AND   THE    MEANS    OF   DIGESTION       29 

this  work  of  separation  is  accomplished  in  the  preparation 
of  food,  both  industrial  and  domestic.  In  this  way  the 
task  of  the  digestive  organs  is  lightened,  and,  as  we  are 
often  told,  overeating  is  made  easy.  Some  foods  are 
quite  devoid  of  residues  when  successfully  digested  and 
absorbed. 

The  early  writers,  having  little  knowledge  of  chemistry, 
were  naturally  led  to  make  much  of  the  mechanical  re- 
duction of  food  in  the  alimentary  tract.  Mastication  sub- 
divides the  food,  and  it  was  held  that  the  later  opera- 
tions, especially  those  of  the  stomach,  were  essentially 
further  grindings  of  a  similar  sort.  Such  mechanical  proc- 
esses as  do  occur  continue  to  be  of  interest,  but  they  are 
now  seen  to  be  preliminary  to  actual  digestion.  More- 
over, we  shall  see  that  it  is  easy  to  assume  that  they  are 
of  a  more  positive  nature  than  is  really  the  case.  The 
human  stomach  is  not  a  mill,  though  the  gizzard  of  a 
bird  may  be  fairly  described  by  that  word. 

In  the  eighteenth  century  the  emphasis  passed  from 
mechanical  factors  to  the  process  of  dissolving  the  food. 
Solution  is  plainly  one  of  the  features  of  digestion,  but  it 
is  a  somewhat  superficial  one.  Of  course,  it  is  natural  to 
believe  that  solid  food  must  become  liquid  before  it  can 
penetrate  the  intestinal  wall,  but  mere  solubility,  as  we 
have  already  seen,  is  not  a  guarantee  of  fitness  for  the 
use  of  the  cells.  Freely  soluble  foods  like  cane-sugar  and 
milk-sugar  require  to  undergo  digestive  changes  just  as 
definite  as  those  carried  out  in  the  case  of  fats  or  coagulated 
proteins.  It  is  often  stated  that  the  object  of  digestion  is 
to  produce  diffusible  substances.  This  statement  is  in- 
adequate, for  diffusibility  like  solubility  does  not  in  itself 
determine  the  utility  of  a  food.  The  sugars  mentioned 
above  are  sufficiently  diffusible,  and  the  changes  which 
they  undergo  before  absorption  serve  a  more  fundamental 
purpose  than  the  mere  hastening  of  their  passage  through 
the  lining  of  the  intestine. 

In  the  light  of  modern  chemical  knowledge  we  can  be 
somewhat  specific  in  regard  to  the  molecular  aspects  of  the 


30  NUTRITIONAL   PHYSIOLOGY 

digestive  processes.  They  are  probably  always  cleavages ^ 
large  molecules  giving  rise  to  smaller.  When  the  original 
molecule  is  of  extraordinary  size,  as  with  proteins  and 
starches,  these  cleavages  have  a  serial  character  and  a 
number  of  intermediate  products  must  accordingly  be 
formed.  That  is  to  say,  the  earlier  products  are  in  turn 
subjected  to  digestion.  Such  cleavages  are  generally,  if 
not  always,  hydrolytic,  that  is,  water  enters  into  the 
reaction  and  its  elements  are  found  combined  in  the 
products.  For  the  simpler  instances  of  digestion,  as  in 
the  case  of  fats  and  of  the  disaccharids,^  we  can  write 
precise  chemical  equations.  We  cannot  do  this  with  the 
same  accuracy  for  the  starches,  and  we  are  still  farther 
from  being  able  to  express  the  exact  manner  in  which  the 
protein  molecule  undergoes  hydrolysis.  Yet  we  have 
sufficient  evidence  that  the  digestion  is  generally  of  a  uni- 
form type. 

Some  constituents  of  the  diet  need  no  digestion.  This 
is  the  case  with  the  mineral  salts,  so  far  as  they  are  ab- 
sorbed, with  the  simple  sugars  (monosaccharids),  and  with 
alcohol.  It  is  hardly  necessary  to  say  that  water  is  also 
ready  for  reception  into  the  body  fluids.  The  numerous 
extractives  are  for  the  most  part  absorbed  in  the  form  in 
which  they  are  eaten.  A  diet  entirely  predigested  seems 
not  to  be  practicable.  If  one  were  prepared  it  would  have 
to  contain  advanced  decomposition  products  of  the  pro- 
teins, which  are  bitter  to  the  taste,  and  an  amount  of  sugar 
which  would  be  cloying  and  subject  to  fermentation. 

Digestion  is  anticipated  to  some  extent  by  changes  in 
our  food  which  precede  its  actual  arrival  in  the  canal. 
The  ripening  of  fruits  and  vegetables,  as  well  as  the  corre- 
sponding processes  in  meat,  illustrate  this  point.  The 
influence  of  cooking  is  not  so  constantly  of  a  sort  to 
initiate  digestion,  yet  in  many  instances  it  is  so.     For 

1  The  follov/ing  equation  illustrates  the  hydrolysis  of  a  disaccharid: 

C12H22O11   +  H2O  =  2C6Hi206. 

This  means  that  one  molecule  of  malt-sugar,  reacting  with  one  mole- 
cule oi  water,  gives  rise  to  two  molecules  of  glucose. 


THE    NATURE    AND   THE    MEANS   OF   DIGESTION       31 

example,  when  meat  is  boiled  the  common  variety  of  con- 
nective tissue  in  it  is  converted  to  gelatin.  This  change 
is  a  typical  hydrolysis,  and  if  it  were  not  executed  in  ad- 
vance it  would  be  an  early  task  of  the  gastric  juice.  When 
cooking  is  attended  by  considerable  drying  of  the  food  it  is 
less  likely  to  count  definitely  toward  digestion.  Most 
proteins  are  coagulated  by  heat,  and  this  change  to  solid 
form  seems  opposed  to  the  general  course  of  events  in  the 
alimentary  system.  The  action  of  microscopic  organisms 
upon  food  substances  is  in  line  more  or  less  with  normal 
digestion;  the  maturing  of  cheese  is  an  example.  But 
bacterial  action  may  depart  so  far  from  the  normal  de- 
composition as  to  generate  products  of  strongly  poisonous 
properties,  the  so-called  ptomains  being  among  them. 

The  Means  of  Digestion. — Hydrolytic  cleavages  closely 
resembling  those  of  animal  digestion  may  be  caused  to 
occur  in  various  ways.  Boiling  food-stuffs  with  acids  ac- 
complishes this.  So  does  treatment  with  alkalis.  Similar 
results  follow  the  application  of  superheated  steam.  But 
the  striking  fact  is  that  such  changes  as  are  brought  about 
in  the  laboratory  by  violent  reagents,  high  temperatures, 
or  both  in  conjunction,  are  caused  to  take  place  in  the 
stomach  and  the  intestine  by  bland  juices  acting  at  the 
mild  temperature  of  the  body.  The  changes  effected  by 
these  juices  are  often  modified  by  the  simultaneous 
activity  of  bacteria,  but  the  presence  of  the  latter  is  to  be 
regarded  as  accidental  and  non-essential. 

The  power  to  digest  foods  has  been  known  for  a  long 
time  to  reside  in  the  secretions  which  enter  the  alimentary 
tract.  It  was  at  first  necessarily  estimated  simply  by 
observing  the  progressive  solution  of  solid  food.  The 
intimate  nature  of  the  process  has  become  appreciated 
more  recently.  Comparison  of  the  juices  from  different 
sources  shows  that  they  are  individual  and  specific  to  the 
extent  that  each  one,  as  a  rule,  acts  upon  certain  classes 
of  food  and  not  on  all.  There  is  sufficient  evidence  for 
the  belief  that  when  a  juice  digests  two  or  more  classes 
of  food-stuffs  it  contains  separate  and  distinct  reagents 


32  NUTRITIONAL   PHYSIOLOGY 

for  the  performance  of  each  Une  of  work.  We  do  not  know 
the  precise  chemical  nature  of  the  active  bodies,  ''  diges- 
tive principles  "  as  they  were  formerly  called,  but  we  know 
a  great  deal  about  the  conditions  of  their  working. 

When  a  digestive  secretion  has  a  single  well-marked 
effect  upon  one  sort  of  food  material  we  say  that  it  contains 
an  enzyme.  We  are  thus  naming  a  body  which  we  know 
chiefly  by  its  power  to  promote  a  certain  chemical  reaction. 
It  is  not  many  years  since  an  able  writer  protested  against 
this  confident  reference  to  a  substance  where  it  is  a  property 
rather  than  a  compound  which  we  are  observing.  It  will 
be  admitted  that  it  is  doubtful  whether  anyone  has  ever 
seen  that  which  is  an  enzyme  and  nothing  else.  What 
we  see  and  handle  are  solutions  possessing  characteristic 
powers  or  dry  preparations  capable  of  furnishing  such 
solutions.  But  it  has  seemed  altogether  reasonable  to 
connect  the  property  with  a  substance  and  we  shall  con- 
tinue to  do  so.  Acting  on  this  basis,  we  say  of  a  juice 
which  hydrolyzes  starch  that  it  contains  a  diastatic  enzyme, 
and  of  one  that  acts  in  a  parallel  fashion  on  proteins  that 
it  contains  a  proteolytic  enzyme.  When  the  action  is 
upon  fats  we  call  the  enzyme  lipolytic,  or,  using  the  sub- 
stantive, we  call  it  a  lipase.  It  is  unfortunate  that  there 
is  much  confusion  at  present  in  the  use  of  such  terms; 
there  are  a  great  many  more  in  use  than  there  need  be. 
The  simplest  plan  is,  perhaps,  to  fall  back  upon  our  Saxon 
and  speak  of  enzymes  as  starch-splitting,  protein-splitting, 
sugar-splitting,  and  fat-splitting.  We  shall  take  pains, 
however,  in  our  detailed  discussion  to  introduce  various 
equivalent  terms.  We  shall  find  especially  that  enzymes 
are  often  named  with  reference  to  their  sources  as  well  as 
to  their  powers. 

Enzymes  are  similar  in  many  respects  to  the  catalyzers 
of  inorganic  chemistry.  Their  presence  accelerates  reac- 
tions which  in  their  absence  might  not  be  appreciable. 
We  do  not  think  of  them  as  contributing  either  material 
or  energy  to  the  process.  They  suggest  the  oil  in  a  machine 
which  lessens  the  resistance  of  its  parts  to  the  driving  force. 


THE    NATURE    AND    THE    MEANS    OF   DIGESTION       33 

The  enzyme  in  a  digesting  mixture  is  not  forcibly  compel- 
ling the  molecules  to  disintegrate,  but  it  is  removing  some 
hindrance  to  their  spontaneous  rearrangement.  It  is  not 
definitely  used  up  in  this  service.  Accordingly,  it  follows 
that  a  limited  quantity  of  a  digestive  juice,  of  course, 
containing  a  still  more  limited  quantity  of  enzyme,  may 
be  responsible  for  an  amount  of  digestion  practically 
unlimited.  Unlimited  time  would  be  demanded  for  such 
a  demonstration.  (This  form  of  statement  should  be 
qualified.  Enzymes  are  somewhat  unstable  and  liable 
to  deteriorate.) 

When  a  process  of  hydrolysis  takes  place  under  the  in- 
fluence of  an  enzyme  and  in  a  glass  vessel,  there  must  be  a 
rising  percentage  of  the  products  and  a  declining  per- 
centage of  the  initial  substance  as  the  reaction  goes  on. 
The  velocity  of  the  transformation  is  found  to  diminish 
and  at  last  it  seems  entirely  arrested.  A  mixture  now  ex- 
ists which  contains  the  first  and  the  last  members  of  the 
chemical  system  in  proportions  which  have  become  con- 
stant. It  is  an  instance  of  chemical  equilibrium.  The 
halting  of  the  reaction  does  not  mean  that  the  enzyme  is 
exhausted.  If  any  means  can  be  devised  by  which  the 
accumulated  end-products  can  be  removed  the  hydrolysis 
will  be  continued.  It  was  specified  above  that  the  trial 
should  be  made  in  a  glass  vessel.  The  reader  will  quickly 
recognize  the  important  difference  between  such  a  con- 
tainer, from  which  nothing  can  escape,  and  the  alimentary 
canal,  from  which  active  absorption  processes  withdraw 
the  products  of  digestion.  A  clear  field  is  thus  provided 
for  the  continuance  to  substantial  completion  of  the  reac- 
tions which  the  enzymes  are  promoting.  The  contrast 
between  laboratory  conditions  and  those  which  prevail 
in  the  body  did  not  escape  the  acute  mind  of  Spallanzani, 
who  was  a  pioneer  among  students  of  these  matters.  As 
early  as  1777  he  recorded  that  the  solution  of  meat  by 
gastric  juice  could  be  greatly  facilitated  by  letting  the 
digestive  fluid  fall  drop  by  drop  upon  the  food  and  to 
trickle  away,  bearing  the  dissolved  products. 
3 


34  NUTRITIONAL   PHYSIOLOGY 

Enzymes  are  exceedingly  sensitive  to  varying  degrees  of 
acidity  and  alkalinity  in  the  medium.  Most  of  them  do 
not  keep  their  efficacy  if  the  solution  is  far  from  the 
neutral  point.  But  they  are  somewhat  individual  in  this 
as  in  other  properties,  the  acid  which  is  highly  favorable 
for  gastric  digestion,  for  example,  being  quite  prohibitive 
of  salivary  action.  They  are  all  destroyed  when  in  solu- 
tion by  temperatures  somewhat  short  of  boiling.  Cold 
suspends  their  activity,  but  does  not  prevent  its  return 
upon  warming.  They  are  most  effective  at  a  temperature 
not  far  from  that  of  the  blood,  though  in  general  a  few 
degrees  higher.  These  relations  between  the  enzymes 
and  temperature  are  much  like  those  established  in  the 
case  of  the  simpler  living  forms.  Having  this  in  mind, 
one  easily  adopts  the  common  practice  of  speaking  of  the 
killing  of  enzymes  by  heat.  It  must  not  be  forgotten  that 
this  is  a  figurative  expression.  We  are  not  justified  in 
thinking  of  enzymes  as  living.  Living  organisms  when 
they  grow  and  multiply  in  a  nutrient  medium  may  de- 
compose it  much  as  suitably  assorted  enzymes  would  do, 
and,  in  fact,  the  organisms  in  question  are  probably  pro- 
ducing their  own  enzymes  for  the  purpose.  Formerly 
such  living  things  as  the  yeasts  and  the  bacteria  were  de- 
scribed as  "  organized  ferments,"  and  the  detached  en- 
zymes, incapable  of  self-multiplication,  were  called 
"  unorganized  ferments."  These  terms  are  not  much  used 
at  the  present  time.  Enzjnnes  are  assumed  to  be  products 
of  living  cells  and  may  be  very  characteristic  fragments  of 
the  cell's  fabric,  but  they  are  not  independently  living. 

The  digestive  changes  to  which  we  pay  most  attention 
are  those  which  occur  in  the  cavity  of  the  alimentary 
canal,  and  which  can  be  observed  to  take  place  also  when 
the  same  mixtures  are  placed  in  the  flasks  and  test-tubes 
of  the  laboratory.  But  we  must  not  overlook  the  prob- 
able fact  that  similar  changes  are  constantly  occurring 
within  the  boundaries  of  every  active  cell.  Intracellular 
digestion,  presumably  made  possible  by  intracellular  en- 
zymes, obviously  takes  place  when  a  protozoan  cell  engulfs 


THE    NATURE    AND    THE    MEANS   OF   DIGESTION       35 

a  solid  food  particle,  and  is  probably  just  as  definite  a 
process  when  a  muscle-fiber  of  the  human  body  nourishes 
itself  at  the  expense  of  the  surrounding  lymph.  We  owe 
to  the  German  chemist,  Abderhalden,  the  clear  exposition 
of  this  second  digestion,  which  is  an  essential  feature  of  the 
life-processes  of  the  higher  animals.  We  shall  recur  to  the 
subject  in  treating  of  protein  metaboHsm. 

While  the  great  majority  of  enzymes  are  hydrolytic  and 
favor  reactions  of  the  class  which  we  have  been  discussing, 
it  must  be  added  that  there  are  other  enzymes  associated 
with  other  orders  of  chemical  change.  Enzymes  which 
promote  oxidations  are  believed  to  play  a  most  important 
part  in  the  activities  of  the  tissues.  When  reactions  are 
hydrolytic  they  proceed  with  but  little  evolution  or 
absorption  of  heat.  When  they  are  oxidative  the  release 
of  energy  is  a  most  characteristic  attendant  condition. 


CHAPTER  IV 
THE  WORK  OF  MUSCLES  AND   GLANDS 

We  cannot  enter  upon  a  description  of  the  alimentary 
canal  and  its  activities  until  we  have  devoted  some  space 
to  the  physiology  of  contraction  and  secretion.  Movement 
is  the  most  familiar  manifestation  of  animal  life.  When 
visible  to  the  naked  eye  it  is  the  expression  of  the  shorten- 
ing of  elongated  units — cells  or  fibers — associated  to  form 
contractile  tissues.  In  the  human  body  there  are  three 
principal  kinds  of  these  tissues.  The  obvious  external 
movements  of  the  limbs  and  the  features,  the  act  of 
breathing,  etc.,  are  produced  by  what  we  call  the  skeletal 
muscles.  Contractile  tissue  of  another  order  forms  the 
walls  of  the  heart  and  furnishes  the  power  for  its  beating. 
A  third  kind  occurs  in  the  walls  of  the  alimentary  tract,  in 
the  blood-vessels,  and  elsewhere. 

The  term  skeletal  applied  to  a  type  of  contractile  tissue 
implies  relationship  to  the  bones.  It  is  easy  to  see  that 
external  movements  are  made  effective  through  the  connec- 
tion of  the  muscles  which  produce  them  with  bones  acting 
as  levers.  In  some  instances  the  term  is  a  misnomer,  for 
there  are  some  small  muscles  histologically  like  the  rest 
which  do  not  act  upon  bones.  This  is  clearly  the  case 
with  the  ring-like  band  which  surrounds  the  mouth  and 
by  its  contraction  puckers  the  lips  as  in  whistling.  The 
large  and  conspicuous  muscles  are  attached,  usually  at 
both  ends,  to  the  bones.  We  can  generally  observe  that 
one  end  is  more  freely  movable  than  the  other.  The  com- 
paratively fixed  end  is  called  the  origin  of  the  muscle,  the 
end  more  subject  to  movement  is  its  insertion. 

What  is  called  a  skeletal  muscle  is  a  bundle  in  which 
we  can  distinguish  an  active  and  a  passive  part.     There  are 

36 


THE    WORK    OF   MUSCLES    AND    GLANDS  37 

the  true  contractile  elements  and  there  is  the  connective 
tissue.  The  inactive  substance  forms  a  sheath  enclosing 
the  rest  and  also  partitions  which  subdivide  the  interior. 
The  arrangement  is  familiar  in  the  cross-section  of  a  piece 
of  meat.  The  subdivision  which  is  apparent  to  the  un- 
aided eye  is  repeated  on  a  microscopic  scale  until  the 
finest  meshes  of  the  connective  tissue  enclose  the  hair-like 
individual  fibers  of  the  muscle.  Each  of  these  slender 
fibers  is  a  miniature  muscle  in  principle.  The  function 
of  the  connective  tissue  is  often  overlooked.  While 
this  part  of  the  muscle  is  entirely  passive  in  character, 
and  scarcely  to  be  considered  alive  excepting  for  a  certain 
power  of  renewal  after  injury,  it  is  quite  necessary  to  the 
act  of  contraction.  It  may  fairly  be  said  to  constitute  a 
harness  through  which  all  the  numberless,  minute  contrac- 
tile elements  are  enabled  to  unite  their  efforts.  As  the 
end  of  a  muscle  is  approached  the  connective  tissue  in- 
creases in  quantity  at  the  expense  of  the  typical  contractile 
material.  In  most  cases  there  is  an  extension  of  the 
muscle  consisting  of  connective  tissue  only,  and  in  a  dense 
form,  which  attaches  the  whole  to  the  bone.  This  is  the 
tendon.  It  may  be  a  long  tough  cord  or  it  may  form  a 
wide  thin  sheet.  A  muscle  deprived  of  its  connective  tis- 
sue would  be  simply  a  mass  of  unattached  living  fibers 
which  might  slip  about  among  themselves,  but  which  could 
not  apply  their  combined  tension  to  accomplish  any  ex- 
ternal effect. 

The  fiber  of  skeletal  muscle  is  a  modified  cell.  Its 
length  is  exceptionally  great  for  its  width,  perhaps  a 
thousand  times  as  great.  When  it  shortens  it  conforms 
to  the  general  principle  laid  down  in  Chapter  I,  that  is,  it 
does  not  diminish  in  volume,  but  only  in  surface  and, 
therefore,  in  length.  How  the  chemical  process  which 
underlies  the  forcible  shortening  is  made  to  contribute 
energy  to  carry  it  out  has  proved  one  of  the  most  difficult 
problems  of  physiology.  It  cannot  be  dealt  with  here. 
But  the  fact  is  to  be  emphasized  that  we  are  in  the  presence 
of  a  mechanism  somewhat  like  the  steam-engine,  inasmuch 


38  NUTRITIONAL   PHYSIOLOGY 

as  it  produces  motion  and  does  physical  work  at  the  cost  of 
fuel  destroyed.  The  resemblance  goes  farther  than  this, 
for  both  with  the  engine  and  with  the  muscle  the  ac- 
complishment of  a  measured  amount  of  work  is  attended 
by  seemingly  wasteful  evolution  of  heat.  An  engine  is 
considered  economical  if  it  turns  15  per  cent,  of  the  energy 
resident  in  its  fuel  into  horsepower.  Muscles  sometimes 
do  better  than  this,  but  much  of  the  time  they  are  even  less 
efficient.  It  is  fair  to  point  out  that  the  heat  set  free  as 
an  accompaniment  of  muscle  contraction  is  often  of  value 
to  the  animal.  In  our  own  case  the  temperature  of  the 
body  must  be  kept  above  that  of  its  usual  surroundings. 
By  far  the  largest  part  of  the  heat  devoted  to  this  main- 
tenance of  a  relatively  high  body  temperature  is  produced 
in  connection  with  muscular  activity.  Muscles  are 
thus  seen  to  be  organs  of  heat  production  as  well  as 
organs  to  carry  out  movements.  When  the  external 
temperature  is  high  or  the  degree  of  muscular  contrac- 
tion is  greatly  above  the  average,  the  heat  evolved 
does  become  distinctly  an  embarrassment  to  the  organ- 
ism. 

The  source  of  the  energy  displayed  in  muscular  activ- 
ity is  chiefly  the  disruption  of  carbohydrate  molecules. 
Sugar  appears  to  be  the  preferred  fuel  of  the  muscular 
machine,  though  other  foods  are  known  to  be  available  also. 
When  sugar  is  completely  oxidized  the  only  end-products 
are  carbon  dioxid  and  water,  the  same  substances  which 
would  be  formed  by  the  literal  burning  of  the  sugar  with 
an  adequate  supply  of  oxygen.  These  waste-products  are 
very  readily  removed  from  the  muscle,  when  its  situation 
is  normal,  by  the  circulating  blood.  The  carbon  dioxid 
will  almost  immediately  escape  from  the  blood  when  it 
passes  through  the  lungs.  The  water  becomes  part  of 
the  large  total  volume  which  is  always  passing  into  and  out 
of  the  body,  and  may  leave  by  all  the  main  channels  of 
excretion — the  respiratory  passages,  the  kidneys,  and  the 
skin.  It  will  be  noted  that  the  quantity  of  water  leaving 
the  body  is  constantly  in  excess  of  the  income.     Ordinar- 


THE    WORK    OF   MUSCLES    AND    GLANDS  39 

ily  the  body  excretes  all  the  water  which  it  receives  plus 
the  water  which  arises  within  it  by  oxidation. 

A  distinction  must  be  borne  in  mind  between  the  com- 
pounds which  for  the  most  part  make  up  the  muscle  and 
those  substances  which  it  is  generally  found  to  use  as 
sources  of  energy.  The  muscle  is  mainly  composed  of 
proteins.  But,  as  just  stated,  it  is  most  apt  to  destroy 
carbohydrates  when  at  work.  One  is  reminded  of  the 
fact  that  a  steam-engine  is  composed  chiefly  of  steel,  but 
burns  coal  as  its  fuel.  The  comparison  is  somewhat 
faulty,  however,  for  it  suggests  a  more  radical  difference 
between  structure  and  fuel  than  we  can  safely  infer 
for  the  muscle.  Under  some  conditions  muscular  work 
may  involve  some  destruction  of  protein  material. 

All  that  has  been  said  of  contraction  up  to  this  time 
applies  equally  to  all  three  classes  of  muscle.  Neverthe- 
less each  type  is  adapted  to  its  particular  work  by  peculiar 
properties.  Skeletal  muscle  is  capable  of  quick  shortening 
and  prompt  relaxation.  A  contraction  may  occur  and 
the  return  to  an  extended  condition  be  accomplished  in 
one-tenth  of  a  second.  The  trained  finger  of  a  pianist  may 
strike  a  key  ten  times  in  a  second.  Such  movements  are 
in  strong  contrast  with  those  executed  by  the  form  of 
muscle  found  in  the  viscera.  The  contractions  of  the 
stomach  develop  very  slowly,  are  maintained  for  some 
time,  and  are  correspondingly  slow  in  fading  out.  Of 
course,  it  is  true  that  skeletal  muscles  may  also  make 
prolonged  contractions,  as  in  keeping  the  body  erect,  carry- 
ing a  suit-case,  and  in  countless  other  instances.  Experi- 
mental study  has  shown  that  such  contractions  as  these  are 
really  compounded  of  successive  brief  twitches  occurring 
too  rapidly  to  permit  relaxation.  In  view  of  this  the 
possibility  of  having  prolonged  contractions  in  skeletal 
muscle  does  not  invalidate  the  statement  that  it  is  essen- 
tially a  quick-acting  tissue. 

Muscle  and  Nerve. — The  conception  that  muscular 
activity  is  due  to  the  nervous  system  is  probably  suffi- 
ciently familiar.     Every  skeletal  muscle  has  its  own  strand 


40 


NUTRITIONAL    PHYSIOLOGY 


of  nerve-fibers  placing  it  in  connection  with  the  brain  or 
the  spinal  cord  and  under  their  control.  If  its  connection 
is  severed  it  becomes  paralyzed  and  remains  inactive, 
unless  special  local  means,  like  electricity,  are  employed 
to  excite  it.  Ordinarily  we  are  justified  in  saying  that 
skeletal  muscle  is  not  automatic,  meaning  that  every  move- 
ment which  it  makes  is  an  indication  of  a  previous  act,  or, 
as  we  say,  a  discharge  on  the  part  of  the  nervous  system. 


Fig.  4. — The  above  represents  somewhat  diagrammatically  a 
very  small  fraction  of  the  length  of  a  fiber  of  skeletal  muscle.  To 
include  the  entire  element  with  the  length  proportional  to  the  width 
we  should  have  to  extend  this  drawing  to  a  length  of  several  yards. 
The  fiber  is  cylindric  and  enclosed  by  a  more  definite  membrane  than 
is  usual  with  animal  cells.  The  cross-marking  is  not  a  feature  of  this 
membrane,  but  stands  for  a  peculiar  organization  of  the  protoplasm 
inside.  Nuclei  are  seen  here  and  there  near  the  surface.  The  seg- 
ment shown  is  supposed  to  be  the  particular  one  about  in  the  middle 
of  the  fiber  within  which  falls  the  connection  with  the  nervous  sys- 
tem. A  nerve-fiber  (n.f.)  is  seen  making  a  junction  with  the  muscle- 
fiber  (M)  through  the  so-called  end-plate  (e.p.). 


In  somewhat  sharp  contrast  is  the  behavior  of  the 
muscle  composing  the  heart  and  of  the  form  which  is 
found  in  the  viscera.  These  two  kinds  of  contractile 
tissue  are  described  as  automatic,  in  the  sense  that  they 
show  a  tendency  to  rhythmic  contraction  and  relaxation 
even  when  deprived  of  their  nervous  connections.  The 
automatic  property  of  the  heart  is  the  cause  of  its  beating. 


THE    WORK   OF   MUSCLES    AND   GLANDS  41 

In  varying  degrees  the  different  portions  of  the  alimentary 
canal  exhibit  the  same  power,  not  ceasing  to  shorten  and 
to  extend  when  observed  entirely  outside  the  body  of  the 
animal.  While  we  emphasize  this  remarkable  tendency 
to  rhythmic  activity,  we  must  hasten  to  add  that  tissues 
showing  such  capacities  are  nevertheless  subject  to  some 
nervous  control.  Thus  the  heart  beats  primarily  because 
of  the  peculiar  nature  of  its  own  substance,  but  varia- 
tions of  rate  and  strength  are  constantly  occurring  as  a 
result  of  the  influence  of  the  nervous  system.  In  this 
connection  it  must  be  pointed  out  that  such  influence  is  not 
necessarily  so  applied  as  to  excite  increased  activity,  but 
may  be  inhibitory,  that  is,  reducing  the  rate  and  force  of 
the  spontaneous  contractions.  A  large  place  is  now  given 
to  the  inhibitory  functions  of  the  nervous  system,  and  we 
shall  meet  with  other  examples  of  the  restraint  which  it 
imposes  upon  various  organs.  A  little  reflection  makes  us 
realize  that  much  of  the  highest  work  of  the  brain  must  be 
in  the  line  of  inhibition.  A  man  is  distinguished  by  the 
acts  from  which  he  refrains  quite  as  much  as  by  those 
which  he  performs. 

Muscular  Tone. — It  will  be  well  before  we  go  farther  to 
make  clear  what  is  meant  by  tone  (tonus,  tonicity)  in 
connection  with  the  behavior  of  contractile  tissues. 
Muscle  is  said  to  exhibit  tone  when  it  is  not  completely 
relaxed.  Tone  is  thus  a  mild,  sustained  contraction.  It 
seems  rarely  to  be  absent  altogether,  but  may  vary  much 
in  degree.  Tone  in  the  skeletal  muscles  gives  them  a  cer- 
tain firmness  and  maintains  a  slight,  steady  pull  upon  their 
tendons.  This  is  not  likely  to  result  in  actual  movement, 
because  these  muscles  usually  fall  into  antagonized 
groups,  one  of  which  opposes  another.  A  heightened  tone 
in  the  muscles  of  the  arm  may  not  change  its  position, 
since  the  force  tending  to  bend  it  may  be  offset  by  an 
equal  tension  adapted  to  straighten  it.  Changes  of  tone 
in  the  walls  of  the  hollow  viscera,  as  the  stomach,  have  a 
much  more  evident  effect,  since  they  alter  the  size  of  the 
cavity.     One  must  discriminate  carefully  between  stretch- 


42  NUTRITIONAL   PHYSIOLOGY 

ing  and  tone  change  in  such  a  case.  A  non-living,  elastic 
sac  may  be  distended  by  increasing  its  contents,  but  will 
react  with  a  pressure  proportional  to  the  distention.  A 
living  organ  which  adapts  itself  to  increased  contents  by  a 
diminution  of  tone  may  exert  no  more  pressure  when  full 
than  when  nearly  empty.  This  principle  is  well  illustrated 
by  the  urinary  bladder.  At  one  time  this  organ  may  have 
a  capacity  of  a  pint,  and  again  its  cavity  may  be  nearly 
obliterated,  but  there  is  no  strict  correspondence  between 
its  size  and  the  internal  pressure.  Indeed,  a  strong  degree 
of  tone  and  a  high  pressure  may  exist  when  the  bladder 
is  quite  small. 

Glands  and  Secretion. — We  have  taken  time  and  space 
to  deal  with  the  elements  of  muscular  activity,  and  we 
must  also  give  a  place  to  another  type  of  tissue  and  to  its 
work.  Some  appreciation  of  the  physiology  of  glands  is  as 
much  a  prerequisite  of  the  study  of  the  alimentary  process 
as  is  a  knowledge  of  the  mechanism  of  contraction.  Every- 
one understands  that  the  nervous  system  throws  the  skele- 
tal muscles  into  their  orderly  activity,  but  the  fact  that 
the  secretions  of  the  body  are  often  produced  under 
nervous  influences  is  not  so  familiar.  Yet  we  do  not  have 
to  look  far  for  suggestive  examples.  The  flow  of  tears 
as  an  accompaniment  of  an  emotional  experience  is  clear 
evidence  that  the  small  organs  above  the  eyeballs  which 
elaborate  the  tears  are  in  connection  with  the  brain  and 
responsive  to  its  changing  conditions.  A  like  relationship 
can  be  demonstrated  for  the  glands  that  produce  saliva 
and  for  those  which  secrete  sweat.  Secretion  and  con- 
traction are  two  manifestations  of  metabolism  which  are 
alike  regulated  by  the  nervous  system.  In  fact,  it  is 
doubtful  whether  we  have  any  other  expression  of  the 
working  of  the  nerve-centers  than  these  two,  the  phe- 
nomena of  consciousness  being  set  aside  for  the  present. 

What,  then,  is  a  gland?  The  word  is  used  sometimes 
to  designate  a  large  organ  like  the  liver,  the  pancreas,  or  a 
kidney.  Sometimes  it  is  used  with  reference  to  a  micro- 
scopic affair  like  an  individual  sweat-gland  or  one  of  the 


THE    WORK   OF   MUSCLES   AND    GLANDS 


43 


minute  pits  in  the  inner  surface  of  the  stomach  or  the  in- 
testine. The  fundamental  structure  is  the  same  in  both 
classes.  The  microscopic  gland  is  a  depression  of  a  cel- 
lular surface — a  pocket,  one  might  say — out  of  which 
when  it  is  active  the  secretion  wells.     The  cells  which 


Fig.  5. — The  principle  of  glandular  structure.  In  the  upper 
figure  a  simple  microscopic  gland  is  supposed  to  be  laid  open  by  a 
section  along  its  vertical  axis.  The  cells  are  seen  to  surround  a  recess 
into  which  they  discharge  their  secretion.  Below,  the  same  struc- 
ture is  shown  in  its  entirety,  and  in  addition  the  encircling  blood- 
vessels which  contribute  to  make  good  the  losses  suffered  by  the 
secreting  cells. 


bound  its  cavity  are  the  producers  of  the  secretion,  and 
are  in  turn  dependent  for  renewal  upon  the  lymph  which 
underlies  them  and  the  blood  which  is  flowing  close  by. 
A  superficial  view  would  suggest  that  such  a  gland  is  a 
filtering  device  adapted  for  straining  off  certain  portions 
of  the  blood.     This  is,  however,  an  entirely  inadequate  con- 


44  NUTRITIONAL   PHYSIOLOGY 

ception.  Most  secretions  contain  substances  which  were 
not  present  in  the  blood  at  all,  and  which  must,  therefore, 
have  been  elaborated  by  the  cells  of  the  gland.  When  we 
remember  that  the  same  blood  flows  through  all  the  glands 
we  cannot  fail  to  be  impressed  by  the  variety  of  the 
products  which  are  made  from  the  same  raw  material — 
products  as  unlike  as  milk  and  bile,  urine  and  saliva. 

A  compound  gland,  like  the  pancreas,  is  an  aggregate 
of  numberless  units,  which  are  individually  like  the  simple 
microscopic  glands.  Within  the  meshes  of  an  abundant 
supporting  tissue  which  is  shot  through  with  blood- 
vessels are  these  small  pockets  walled  around  with  the 
characteristic  cells  of  the  gland.  These  ultimate  recesses 
are  called  alveoli  or  acini.  Each  has  a  way  open  through 
which  its  liquid  product  may  move  toward  an  outlet. 
Usually  there  is  a  single  main  duct  formed  by  the  union 
of  all  the  fine  passages  from  the  alveoli  and  bearing  their 
combined  contributions.  A  compound  gland  may  have 
more  than  one  duct.  Glandular  secretions  may  be 
discharged  directly  upon  the  surface  of  the  skin,  as  in  the 
case  of  the  sweat,  or  they  may  enter  cavities,  as  happens 
with  the  gastric  juice,  the  pancreatic  juice,  and  the  secre- 
tion of  the  intestinal  lining.  The  bile  and  the  urine  are 
two  secretions  which  accumulate  temporarily  in  special 
containers,  the  gall-bladder  and  the  urinary  bladder 
respectively,  before  they  reach  their  final  destination. 

Internal  Secretions. — It  may  not  be  premature  to  add 
at  this  point  that  any  organ  may  yield  some  peculiar  prod- 
uct of  its  own  life  process  to  the  lymph  or  to  the  blood  as 
well  as  to  the  cavities  of  the  hollow  viscera  and  to  the 
exterior  of  the  body.  A  product  of  this  kind  which 
merges  with  the  circulating  medium  instead  of  appearing 
distinct  and  separate  from  it  is  called  an  internal  secretion-. 
One  may  maintain  that  every  organ  has  such  a  secretion, 
for  inasmuch  as  each  has  its  unique  chemical  composition 
and  its  distinctive  metabolism,  it  must  give  to  the  blood 
compounds  which  no  other  organ  duplicates.  As  stated 
before,  the  actual  make-up  of  the  blood  is  the  resultant 


THE    WORK    OF    MUSCLES    AND    GLANDS  45 

of  the  action  of  all  the  tissues  upon  it.  But  we  shall  find 
that  internal  secretion  is  a  function  much  more  clearly 
attributable  to  certain  organs  than  toothers  and  most 
evident  in  connection  with  a  few  small  structures  like  the 
thyroid  and  the  adrenal,  to  which  later  reference  must 
be  made.  To  have  an  internal  secretion  an  organ  need 
not  be  a  typical  gland.  No  duct  will  be  required  to  carry 
such  materials  as  its  cells  turn  over  to  the  blood-stream. 
In  some  cases  organs  believed  to  work  along  these  lines  are 
spoken  of  as  ductless  glands.  A  word  recently  introduced 
as  an  equivalent  of  the  term  "  internal  secretion"  should 
be  given.  It  is  the  word  ^'  hormone,"  meaning  a  chemical 
messenger,  a  very  convenient  and  suggestive  expression. 

Absorption  and  Secretion.— Gland-cells  have  been  said 
to  draw  upon  the  blood  or  the  lymph  for  their  raw  mate- 
rial and  to  manufacture  their  secretions  therefrom.  In 
this  process  something  enters  the  deeper  boundary  of  the 
cell  layer  and  in  a  more  or  less  transformed  state  it  is 
later  discharged  from  the  exposed  surface.  It  is  helpful 
to  compare  this  operation  with  what  takes  place  in  the 
intestine  when  the  products  of  the  digestive  cleavage  are 
being  removed  to  the  circulation.  When  absorption  is 
going  on  it  is  the  exposed  ends  of  the  cells  which  receive 
dissolved  substances  and  their  deeper  borders  which  are 
discharging  to  the  fluids  that  underlie  them.  Such  a 
process  has  been  well  called  "  reversed  secretion,"  and 
there  is  the  same  possibility  of  an  extensive  making  over 
of  the  transferred  material  in  this  case  as  in  the  other. 
In  other  words,  the  digestive  products  which  are  last 
detected  in  the  intestine  are  not  necessarily  those  which 
will  be  dealt  out  to  the  blood  by  the  cells  of  the  absorbing 
membrane.  Both  secretion  and  absorption  are  phenom- 
ena which  can  be  completely  carried  out  only  by  living  cells. 
Each  is  probably  promoted  by  a  definite  application  of 
energy  on  the  part  of  the  cells  concerned.  In  either  case 
it  is  possible  that  there  may  be  some  transfer  of  material 
through  the  clefts  between  the  cells  as  well  as  through 
the  cell  bodies. 


CHAPTER  V 
REFLEX  ACTION 

In  the  previous  chapter  it  was  pointed  out  that  all  the 
work  done  by  the  skeletal  muscles  is  in  response  to  the 
discharges  of  the  central  nervous  system.  For  the  other 
types  of  muscle — the  cardiac  and  the  visceral — it  was 
shown  that  there  is  an  inherent  tendency  to  rhythmic 
activity,  but  that  over  these  tissues  also  the  nervous  sys- 
tem exercises  a  regulation.  Finally,  it  was  stated  that  the 
glands  likewise  are  subject  to  central  government,  al- 
though not  to  the  same  degree  in  all  cases.  We  must 
now  proceed  to  consider  how  the  nerve-centers  are  them- 
selves prompted  to  throw  muscles  and  glands  into  action. 

As  we  observe  the  body  at  work  we  cannot  fail  to  be 
impressed  with  the  timeliness  of  its  adjustments.  It  is 
constantly  meeting  with  emergencies  and  adapting  itself 
to  new  conditions.  If  we  are  inclined  to  attribute  all 
these  quick  adaptations  to  intelligent  choice  of  courses 
to  be  pursued  we  shall  find  that  we  cannot  long  defend 
such  an  explanation  of  the  facts  as  they  occur.  We  can- 
not pretend  that  we  think  of  each  inequality  of  the 
pavement  as  we  cross  the  street,  or  of  each  individual  in 
the  crowd  through  which  we  make  our  way.  The  balan- 
cing of  our  bodies,  standing  or  walking,  is  not  a  matter 
about  which  we  are  given  to  deliberating.  These  things 
seem  to  take  care  of  themselves.  It  is  such  adjustments 
which  "  seem  to  take  care  of  themselves  "  that  are  called 
reflex  actions.  A  reflex  is  an  adaptive  change  to  meet 
some  new  external  condition  brought  about  through  the 
agency  of  the  central  nervous  system.  We  may  or  may 
not  notice  the  occurrence  of  a  reflex.  If  consciousness 
is  at  all  involved,  it  is  incidental  and  not  causal.     Often 

46 


REFLEX    ACTION  47 

the  conscious  effort  is  rather  to  prevent  the  reflex  from 
taking  place,  as  is  apt  to  be  true  when  we  sneeze.  Of 
some  reflexes  we  are  quite  unhkely  to  be  aware;  this  is  the 
case  with  the  narrowing  of  the  pupil  in  response  to  in- 
crease of  light. 

Let  us  now  go  into  some  detail  and  analyze  carefully 
the  reflex  process.  We  have  seen  that  the  primary  cause 
is  an  external  change  of  some  sort,  the  word  "  external  " 
meaning  outside  the  central  nervous  system  and  not 
necessarily  outside  the  body.  The  change  which  is  at  the 
root  of  the  reflex  is  usually  referred  to  as  an  external 
stimulus.  It  would  be  easy  to  give  a  long  list  of  exam- 
ples. A  foreign  particle  comes  in  contact  with  the  larjnix; 
its  contact  is  the  stimulus  which  develops  the  coughing 
reflex.  Slight  drying  of  the  exposed  surface  of  the  eyeball 
is  a  common  cause  of  the  winking  reflex.  Irritation  of  the 
lining  of  the  stomach  is  the  most  frequent  of  the  many 
possible  stimuli  through  which  vomiting  can  be  excited. 

External  stimuli  would  fail  of  any  extended  effect  if  it 
were  not  for  the  nervous  connections  of  the  parts  affected. 
In  the  last  chapter  the  nervous  system  was  spoken  of  as 
sending  its  impulses  out  to  muscles  and  glands.  But 
its  work  is  twofold.  It  not  only  acts,  but  it  is  acted  upon. 
Its  fibers  fall  into  two  classes,  those  which  are  concerned 
with  transmission  of  effects  outward  from  the  brain  and  the 
spinal  cord,  and  those  having  the  opposite  function,  the 
carrying  inward  of  impulses  started  by  external  causes. 
The  first  class  of  conductors  are  usually  called  motor;  the 
second,  sensory.  Both  terms  are  open  to  objection,  as  a 
little  consideration  will  show.  The  effects  which  the  ner- 
vous system  produces  in  the  tissues  of  the  body  are  not 
solely  movements.  The  word  "  motor,"  then,  is  not  in- 
clusive enough.  It  is  better  to  substitute  the  word 
efferent,^  which  means  simply  centrifugal,  and  which  im- 
plies nothing  whatever  about  the  nature  of  the  responses 
evoked.  Efferent  fibers  may  be  motor,  that  is,  exciting 
contraction,  but  they  may  also  inhibit  contraction,  and 
*  Efferre,  to  bear  away. 


48  NUTRITIONAL   PHYSIOLOGt 

when  they  end  in  connection  with  the  cells  of  glands  they 
may  be  secretory.   Probably  also  they  may  inhibit  secretion. 

Just  as  we  have  found  the  word  "  motor  "  inadequate 
and  have  agreed  to  replace  it  by  "  efferent,"  so  the  word 
*'  sensory  "  does  not  properly  indicate  the  whole  service  of 
the  fibers  which  bear  impulses  toward  the  brain  and  cord. 
Sensory  implies  "  productive  of  sensation/'  and  we  cannot 
assign  such  a  property  to  all  the  two  million  fibers  which 
assail  the  centers  with  their  communications.  In  the 
great  majority  of  cases  we  do  not  feel  any  consequences  of 
their  activity.  The  term  afferent  is  free  from  this  objec- 
tion and  is  the  logical  complement  of  efferent.  If  one 
hastens  to  ask  what  is  the  significance  of  afferent  fibers 
which  do  not  arouse  sensation,  the  answer  is  simple  and 
definite:  They  produce  reflexes. 

If  the  first  element  in  the  reflex  process  is  the  applica- 
tion of  an  external  stimulus,  it  is  now  clear  that  the  next 
element  is  the  afferent  transmission  of  the  impulses. 
What  these  impulses  are  cannot  be  discussed.  It  should 
be  recalled  that  they  are  not  fluid  pulses  nor  electric  cur- 
rents in  the  usual  sense  of  the  expression.  They  repre- 
sent energy  of  some  kind  in  rapid,  but  not  immeasurably 
rapid,  motion.  They  pass  along  the  nerves  at  rates  in 
excess  of  100  feet  in  a  second,  so  that  the  longest  paths  in 
the  human  body  are  traversed  almost  instantaneously. 
The  time  used  in  such  transits  might  be  quite  appreciable 
if  we  could  observe  it  closely  in  a  whale. 

When  the  afferent  impulses  reach  the  central  nervous 
system  the  third  event  in  the  development  of  the  reflex 
act  occurs.  This  is  localized  in  the  brain  or  the  spinal 
cord,  and  we  may  speak  of  it  as  ''  a  central  process  " 
without  committing  ourselves  as  to  its  exact  character. 
What  we  actually  observe  is  that  the  arrival  of  the  afferent 
impulses  is  followed  by  the  appearance  of  efferent  ones. 
It  is  not  necessary  to  decide  whether  these  efferent  im- 
pulses are  the  same  currents  which  just  entered  the  in- 
tricate fabric  of  the  central  organ  and  which  have  found  a 
path  open  through  its  mazes  which  has  led  them  out 


REFLEX    ACTION 


49 


again.  That  is  one  way  of  picturing  the  phenomenon. 
According  to  the  older  and  more  famiUar  view  the  impulses 
which  come  out  are  not  those  which  went  in,  but  a  new 
set  generated  by  an  energetic  metabolic  process,  a  discharge 
on  the  part  of  cells  in  the  brain  or  the  cord.  If  this  is  the 
true  conception  the  afferent  impusles  serve  to  "touch  off" 
irritable  nervous  elements,  much  as  these  elements  in  their 
turn  may  touch  off  muscle-fibers  or  gland-cells. 


Fig.  6. — The  principle  of  reflex  action.  The  subject  touches  a  hot 
object  {H).  Afferent  nerve-impulses  travel  the  route  marked  by- 
dots  and  dashes  to  the  spinal  co];d  (S).  Efferent  impulses  return 
promptly  along  the  route  marked  by  little  crosses  to  the  muscle  (M), 
which  co-operates  with  others  not  shown  to  withdraw  the  finger 
from  the  stimulating  surface.  The  situation  of  the  co-ordinating 
center  is  left  undetermined,  whether  in  the  brain  or  the  cord. 

The  fourth  step  in  the  evolution  of  the  reflex  is  the 
efferent  transmission.  This  may  be  said  always  to  be  more 
voluminous  than  the  afferent  flow  which  went  before. 
Impulses  go  out  by  many  channels,  where  but  few  were 
engaged  in  bringing  them  in.  A  great  characteristic  of 
the  "central  process"  is  the  spreading  of  the  initial  stimu- 
lation, so  that  there  seems  to  be  no  proportion  between  the 


50  NUTRITIONAL   PHYSIOLOGY 

cause  and  the  response.  The  number  of  nerve-fibers 
which  can  be  excited  by  the  slender  proboscis  of  a  mosquito 
as  it  pierces  the  skin  of  a  sleeper  must  be  very  small.  The 
reflex  movement  which  results  may  involve  a  very  large 
share  of  his  skeletal  muscles. 

The  fifth  and  final  occurrence  completing  the  reflex  is 
the  reaction  on  the  part  of  the  muscles  or  the  gland-tissue 
in  which  the  efferent  fibers  end.  As  already  indicated,  this 
may  be  a  movement,  an  outpouring  of  secretion,  or  it  may 
have  a  negative  character,  the  suppression  of  movements 
that  would  naturally  have  occurred,  or  possibly  the  with- 
holding of  some  secretion  which  would  otherwise  have  been 
discharged.  The  illustrations  of  reflex  action  most  often 
chosen  are  those  in  which  an  immediate,  even  abrupt, 
response  is  seen.  Yet  it  is  quite  easy  to  find  examples 
of  gradual  adjustment  to  the  new  external  condition. 
Changes  of  color,  the  outward  sign  of  changes  in  the 
blood-supply  of  the  skin,  when  they  occur  on  account  of 
warming  or  cooling  of  the  surface,  are  reflexes  of  this 
prolonged  and  gentle  order. 

If  there  is  any  doubt  as  to  whether  a  certain  action  is  to 
be  classed  as  a  reflex,  it  may  be  tested  according  to  the 
foregoing  analysis.  There  must  be  an  assignable  stimulus, 
external  at  least  as  regards  the  central  nervous  system, 
there  must  be  an  afferent  flow  of  the  impulses  resulting 
from  the  stimulation,  a  process  within  the  bounds  of  the 
central  axis,  a  return  flow  of  impulses  in  multiplied  volume, 
and  the  action  itself.  The  more  one  thinks  of  the  common 
course  of  events,  the  larger  the  number  of  actions  which 
he  finds  he  can  place  in  this  class.  It  becomes  appropriate 
to  ask  what  kinds  of  bodily  activity  are  outside  this  depart- 
ment. To  this  question  it  can  be  replied  that  automatic 
actions,  such  as  the  beating  of  the  heart,  are  to  be  dis- 
tinguished from  reflexes.  The  nervous  system  is  not  re- 
quired to  maintain  the  heart-beat.  There  are  cases  also 
in  which  the  chemical  composition  of  the  blood  reaching 
the  centers  modifies  their  behavior  and  causes  them  to  send 
out  certain  impulses.     Such  cases  do  not  fit  our  descrip- 


REFLEX    ACTION  51 

tion  of  the  reflex,  since  in  them  the  stimulation  is  appUed 
centrally  and  no  afferent  nervous  mechanism  is  needed. 
Our  breathing  movements  are  determined  to  a  great  extent 
by  such  chemical  conditions,  but  it  is  a  fact  that  reflex 
disturbances  of  the  breathing  are  so  prevalent  that  it  is 
often  difficult  to  give  just  recognition  to  the  two  factors. 

We  are  accustomed  to  contrast  sharply  actions  which 
are  reflex  with  those  which  we  regard  as  strictly  volun- 
tary or  deliberate.  The  distinction  is  a  convenient  one 
and  not  generally  productive  of  confusion,  but  sometimes 
it  becomes  quite  difficult  to  draw  the  line.  It  may  be 
urged  that  all  our  conscious,  intentional  acts  are  performed 
in  answer  to  external  conditions  which  have  risen  to  make 
an  occasion  for  such  new  adjustments.  So  it  might  be 
argued  that  the  writing  of  a  word  from  a  copy  should  be 
considered  a  reflex  in  which  the  retinal  image  of  the  copy 
furnished  the  external  stimulus.  Such  images  printed 
upon  the  retina  of  the  trained  pianist  by  the  notes  that  are 
before  him  cause  his  fingers  to  drop  upon  the  corresponding 
keys  of  the  instrument.  It  may  be  claimed  that  this  is  a 
reflex  action.  Without  denying  the  force  of  such  reasoning, 
we  shall  do  well  to  restrict  the  term  to  the  class  of  responses 
for  which  we  are  quite  sure  that  attention  is  unnecessary, 
and  usually  to  those  for  which  we  have  an  inborn  or  at 
least  a  very  early  developed  capacity.  When  we  ask 
ourselves  whether  any  act  is  really  other  than  the  result 
of  external  circumstances  affecting  an  organism  with  its 
own  past  history  registered  in  its  structure,  we  find,  al- 
most with  a  shock,  that  we  are  face  to  face  with  philo- 
sophic and  ethical  problems,  responsibility  and  free  will. 
Most  of  us  like  to  believe  that  a  place  is  to  be  reserved  for 
a  type  of  action,  even  though  it  may  be  rare  and  slight, 
which  is  not  externally  caused. 

The  great  difficulty  encountered  by  the  beginner  in 
physiology  lies  in  the  attempt  to  realize  the  inevitable 
character  of  reflexes  and  their  structural  basis.  He  finds 
it  hard  not  to  read  conscious  purpose  into  acts  which  so 
constantly  prove  advantageous  to  the  individual.     When  a 


52  NUTRITIONAL   PHYSIOLOGY 

frog  adroitly  catches  a  fly  it  is  natural  to  assume  a  desire 
and  a  design  on  the  part  of  the  frog.  Scientific  analysis 
nevertheless  makes  it  appear  far  more  probable  that  the 
fly  is  entrapped  because  the  frog  is  a  mechanical  device 
adapted  to  do  this  thing  over  and  over  again.  The  eye 
receives  the  flitting  shadow  of  the  insect,  the  stimulus 
excites  the  brain,  and  the  well-directed  fling  of  the  tongue 
follows.  The  reflex  is  not  done  away  with  when  the  part 
of  the  brain  most  likely  to  stand  in  relation  to  consciousness 
has  been  destroyed.  We  have  to  remember  that  much  of 
the  service  of  the  eye  is  subconscious,  as  when  it  makes  us 
turn  aside  from  obstacles  in  our  path.  It  is  in  this  way 
that  the  eyes  of  the  somnambulist  assist  in  guiding  his 
movements.  Conscious  attention  is  no  more  essential  to 
such  a  use  of  the  eyes  in  the  waking  than  in  the  abnormal 
sleeping  state.  In  fact,  close  attention  to  the  balancing 
of  the  body  is  quite  as  likely  to  derange  as  to  promote  the 
reflex  adjustment. 

Central  Resistance. — Reflexes  are  not  obtainable  with 
the  same  ease  at  all  times.  We  express  this  fact  by  saying 
that  there  are  variations  of  resistance  in  the  central  ner- 
vous system.  If  reflexes  are  hard  to  bring  about,  we  say 
that  the  resistance  is  high ;  if  they  occur  with  unusual  free- 
dom and  seem  disproportionate  to  the  exciting  stimuli,  we 
say  that  the  resistance  is  low.  Narcotics  and  anesthetics 
are  said  to  raise  the  resistance,  and  their  effect  can  be 
gaged  by  observing  the  degree  of  difficulty  with  which 
certain  reflexes  can  be  produced,  or  whether,  indeed,  they 
can  be  produced  at  all.  Drugs  of  an  opposite  order,  the 
true  stimulants,  make  it  easy  to  call  out  most  reflexes. 
When  one  is  distinctly  under  the  influence  of  coffee,  a  noise 
may  cause  one  to  start,  with  a  sharp  contraction  of  many 
muscles.  The  auditory  stimulus  has  an  undue  effect,  and 
it  is  natural  to  assume  that  the  conditions  in  the  brain  and 
cord  are  uncommonly  favorable  to  the  penetration  and  to 
the  multiplication  of  nervous  impulses.  In  poisoning  by 
strychnin  such  an  extension  of  conduction  may  exist  that 
some  trifling  cause  may  precipitate  a  terrific  and  exhaust- 


REFLEX   ACTION  53 

ing  convulsion.  Clearly,  then,  a  certain  degree  of  central 
resistance  is  the  most  favorable  condition  for  the  activities 
of  life.  Any  increase  will  tend  to  prevent  needed  adapta- 
tions to  external  changes,  and  any  great  decrease  will  make 
the  reflex  responses  exaggerated,  disorderly,  and  ill  suited 
to  their  object.  There  is  reason  to  suppose  that  the  more 
frequently  occurring  reflexes  become  easier  of  production 
through  a  lowering  of  resistance  in  their  particular  path- 
ways. This  brings  us  close  to  the  subject  of  habit  for- 
mation. 

Our  emphasis  has  been  constantly  upon  the  advantage 
derived  by  the  animal  (or  by  man)  from  the  possession  of 
reflex  capacities.  When  the  environment  is  the  accus- 
tomed one  and  the  changes  taking  place  are  such  as  the 
species  has  often  experienced,  we  find  that  almost  every 
reflex  is  obviously  beneficial.  The  reactions  are  such  as 
maintain  bodily  equilibrium,  secure  nutriment,  evade  or 
defeat  enemies,  resist  changes  of  temperature,  all  making 
for  self-preservation.  But  it  must  be  noted  that  an  unin- 
telligent mechanism  will  act  amiss  in  any  environment 
which  is  suflftciently  imlike  the  accustomed  one.  It  will 
hardly  be  claimed  that  the  reflexes  exhibited  by  the  novice 
on  first  going  to  sea  help  him  in  the  struggle  for  existence. 
A  number  of  reflex  effects  can  be  thought  of  which  can 
scarcely  be  of  value.  Sneezing  when  going  out  into  bright 
sunlight  is  one  of  these.  Hiccups  following  immoderate 
laughter  do  not  seem  to  be  of  any  service,  nor  does  laughter 
itself  when  induced  by  tickling.  These  instances,  which 
on  the  whole  have  little  importance,  are  mentioned  simply 
to  enforce  the  contention  that  the  reflex  mechanism,  how- 
ever refined,  is  not  directed  in  its  routine  performances 
by  intelligence.  Its  structure  determines  its  conduct.  The 
finger  laid  upon  hot  iron  is  twitched  away  before  the  situa- 
tion is  reasoned  out,  in  fact,  before  pain  is  felt.  Central 
connections  exist  which  make  the  movement  sure  to  occur. 
If  we  could  rearrange  those  central  connections  we  can 
conceive  of  a  luckless  subject  who  would  not  remove  his 
finger  from  the  stove,  but  would  stand  violently  coughing 
while  the  injury  proceeded. 


CHAPTER  VI 
THE  ALIMENTARY  CANAL 

The  single-celled  animal  digests  its  food  within  its  own 
protoplasm,  sometimes  holding  it  for  a  while  in  a  temporary 
cavity  filled  with  fluid,  the  so-called  food  vacuole.  In 
such  intracellular  cavities  true  digestive  secretions  contain- 
ing enzjnnes  are  doubtless  at  work.  It  is  probable  that 
single-celled  forms  may  also  secrete  enzymes  to  the  exte- 
rior and  so  modify  food  material  which  is  near-by,  but  not 
yet  enclosed.  This  appears  to  be  the  case  with  bacte- 
ria when  they  dissolve  the  solid  gelatin  in  which  they  are 
growing. 

Among  many-celled  animals  digestion  of  this  second 
type,  that  is,  external  to  the  cells,  becomes  more  con- 
spicuous. Their  bodies  are  so  formed  as  to  contain  spaces 
in  which  food  may  undergo  digestion  and  from  which  the 
hydrolyzed  products  may  be  absorbed.  In  the  sea- 
anemone  a  round  opening  or  mouth  leads  to  a  cavity  which 
is  very  large  in  proportion  to  the  size  of  the  animal. 
This  primitive  alimentary  tract  has  no  other  opening. 
In  the  earthworm,  a  somewhat  more  highly  developed 
form,  a  straight  canal  in  the  axis  of  the  body  leads  from 
a  mouth  near  the  anterior  end  to  an  anus  at  the  posterior. 
However  much  the  alimentary  systems  of  the  higher  ani- 
mals may  be  elaborated,  each  still  represents  a  more  or  less 
winding  passage  between  a  mouth  through  which  food  is 
received  and  a  vent  or  anus  for  the  discharge  of  residues  and 
excretions.  The  canal  may  be  greatly  lengthened  through 
coiling.  Some  sections  may  be  widened  and  others  nar- 
rowed ;  the  walls  in  some  places  may  be  thick  and  elsewhere 
thin.  Local  differentiation  of  this  kind  causes  us  to  dis- 
tinguish in  the  human  subject  the  familiar  divisions  of  the 

54 


THE    ALIMENTARY   CANAL 


55 


tract,  as  the  esophagus,  the  stomach,  the  small  and  the 
large  intestines.  The  lengthening  of  the  system,  it  should 
be  noted,  does  not  merely  increase  its  capacity,  but  multi- 
plies the  surface  available  for  the  processes  of  absorption. 
A  few  anatomic  expressions  may  well  be  defined  at  this 
time.     Anterior,  as  Ave  shall  use  the  word,  means  toward 


II 


III 

Fig.  7. — I  represents  a  protozoan  cell — an  ameba — which  has 
enclosed  a  particle  available  for  food  (F).  The  particle  occupies 
the  center  of  a  clear  space  or  vacuole  (T).  Undoubtedly  it  is  sur- 
rounded by  a  fluid  having  digestive  powers.  II  is  a  diagrammatic 
section  through  the  familiar  sea-anemone.  There  is  a  relatively 
huge  digestive  cavity  (S)  with  a  single  opening  to  the  exterior  (M). 
Ill  suggests  the  type  of  alimentary  system  found  in  the  earthworm 
and  in  higher  animals.  Two  openings  exist,  the  mouth  (M),  de- 
finitely devoted  to  the  reception  of  food,  and  the  anus  {A),  used 
exclusively  for  the  discharge  of  wastes. 


the  head;  posterior,  away  from  the  head.  Dorsal  means 
toward  the  back;  ventral,  toward  the  front.  Right  and 
left  have  their  ordinary  use.  (In  most  figures,  the  subject 
being  viewed  from  in  front,  right  and  left  are  reversed.) 
Reference  must  often  be  made  to  the  body  cavities.     These 


56  NUTRITIONAL   PHYSIOLOGY 

are  the  thoracic  cavity  above  the  diaphragm  and  within 
the  cage  of  the  ribs,  the  abdominal  cavity  below  the  dia- 
phragm, and  the  much  smaller  pelvic  cavity  bounded  by 
the  bones  of  the  hip  girdle.  When  we  speak  of  these  as 
cavities  we  do  not  mean  that  they  contain  any  air-filled 
space.  They  are  completely  filled  by  the  organs  which 
they  enclose  plus  a  small  quantity  of  fluid.  Hence  they  are 
only  potential  cavities  in  life,  becoming  actual  when  their 
contents  are  removed  in  course  of  dissection.  The  thoracic 
cavity  contains  the  lungs,  nearly  surrounding  the  heart, 
and  is  traversed  by  the  esophagus.  The  abdominal  cavity 
is  filled  almost  entirely  by  the  organs  of  digestion — the 
stomach,  the  small  and  large  intestines,  the  liver,  and  the 
pancreas.  The  spleen  at  the  left  of  the  stomach  is  less 
certainly  connected  in  its  functions  with  the  alimentary 
system.  The  kidneys  lie  in  the  dorsal  body  wall  rather 
than  in  the  abdomen.  In  the  small  pelvic  cavity  are  the 
urinary  bladder,  the  terminal  part  of  the  large  intestine, 
and  the  reproductive  organs. 

The  mouth,  the  first  division  of  the  alimentary  canal, 
scarcely  calls  for  detailed  description.  Above,  a  bony  par- 
tition separates  it  from  the  intricate  spaces  of  the  nasal 
passages.  At  the  back  this  ''roof"  is  prolonged  as  a 
mobile,  muscular  curtain — the  soft  palate.  Below  the 
edge  of  the  soft  palate  a  region  is  reached  which  is  common 
to  the  alimentary  and  respiratory  systems.  This  segment 
of  the  canal  is  known  as  the  pharynx,  though  the  term  is 
extended  also  to  the  space  behind  the  soft  palate,  which  is 
above  the  normal  course  of  food.  The  teeth  and  the 
tongue  with  its  wonderful  muscular  development  are  suffi- 
ciently obvious.  Ducts  from  the  salivary  glands  open 
into  the  mouth.  We  are  rarely  conscious  of  the  situation 
of  these  openings,  though  in  the  dentist's  chair  we  may 
notice  the  rapid  flow  of  saliva  from  one  which  is  opposite 
the  upper  molars.  This  is  the  place  of  entrance  of  the 
secretion  of  the  parotid  gland,  situated  before  and  below  the 
ear,  the  gland  usually  affected  in  mumps.  Under  the 
tongue  and  within  the  sweep  of  the  lower  jaw-bone  there 


Fig.  8. — The  human  aUmentary  canal  shown  diagrammatically : 
0  is  the  esophagus;  S  is  the  stomach;  S.I.  suggests  the  small  in- 
testine; C  is  the  colon  (see  Fig.  14) ;  R  is  the  rectum.  The  connection 
between  the  stomach  and  the  small  intestine  occurs  behind  the 
transverse  colon,  which  also  hides  the  pancreas. 


Fig.  9. — Relations  of  the  mouth  and  nose.  This  is  a  vertical  sec- 
tion through  one  nostril,  and  therefore  slightly  away  from  the 
midplane  of  the  head.  The  convoluted  character  of  the  lateral 
wall  of  the  nasal  cavity  is  suggested.  The  connection  between 
the  nose  and  the  throat  will  be  seen  behind  the  soft  palate  (P); 
L  is  placed  in  the  iarjmx,  above  which  is  shown  the  spur  of  the  epi- 
glottis; O  indicates  the  course  of  the  esophagus.  It  will  be  noted 
that  the  course  taken  by  the  food  crosses  the  route  of  the  breathing 
m  the  pharynx. 


THE    ALIMENTARY    CANAL  57 

are,  on  either  side,  two  other  glands,  the  submaxillary  and 
the  sublingual,  with  ducts  opening  in  the  floor  of  the 
mouth. 

Below  the  root  of  the  tongue  there  is  a  leaf -like  pro- 
jection, the  epiglottis,  which  juts  backward  and  guards  the 
entrance  to  the  larynx.  Through  the  larynx  a  way  is  open 
to  the  trachea  and  the  lungs.  At  this  point,  therefore, 
the  courses  taken  by  the  food  and  by  the  breath  part  com- 
pany. From  here  the  esophagus  extends  through  the  neck 
and  the  thorax,  lying  at  first  behind  the  trachea,  and  lower 
do\Mi  passing  back  of  the  heart.  Perforating  the  dia- 
phragm slightly  to  the  left  of  the  midline  it  opens  into  the 
stomach. 

The  stomach  is  the  most  expanded  part  of  the  aliment- 
ary canal.  Its  position  is  higher  up  than  is  generally  as- 
sumed, so  that  it  is  well  within  the  embrace  of  the  lower 
ribs  on  the  left  side.  It  has  a  capacity  varying  greatly 
with  the  degree  of  its  distention  and  with  its  variations 
of  tone.  After  a  full  meal  it  may  contain  more  than  a 
quart.  The  form  of  the  stomach  also  changes  consider- 
ably from  time  to  time,  but  we  distinguish  a  large,  rounded 
portion  toward  the  left  and  a  more  conical  region  tapering 
off  toward  the  right  and  joining  the  small  intestine.  The 
opening  from  the  esophagus  into  the  stomach  is  called  the 
cardia,  and  that  from  the  stomach  to  the  small  intestine  is 
the  pylorus.  The  pylorus  is  a  trifle  to  the  right  of  the  mid- 
line. The  upper  border  from  the  cardia  to  the  pylorus  is 
the  ^'lesser  curvature"  of  the  stomach;  a  line  drawn  from 
the  cardia  around  the  convex  left-hand  side  and  thence 
along  the  lower  margin  to  the  pylorus  is  said  to  follow  the 
''greater  curvature." 

Leading  away  from  the  pylorus  the  small  intestine  de- 
scribes a  short  turn,  within  which  is  the  pancreas.  This 
first  curve  is  called  the  duodenum.  The  remainder  of  the 
small  intestine  is  a  slender  tube  almost  20  feet  in  length, 
coiled  upon  itself  in  a  confusing  manner.  Two  divisions 
are  recognized,  the  jejunum,  continuous  with  the  duodenum 
and  the  ileum,  extending  onward  to  join  the  large  in- 


58  NUTRITIONAL   PHYSIOLOGY 

testine.  No  sharp  line  of  demarcation  exists  between 
these  sections,  but  the  latter  is  regarded  as  constituting 
somewhat  more  than  half  the  whole.  The  ileum  finally 
arrives  at  a  point  not  far  from  the  crest  of  the  right  hip- 
bone and  there  enters  the  large  intestine. 

The  large  intestine  is  so  called  from  its  diameter,  which 
is  two  or  three  times  that  of  the  small.     It  is  quite  as  often 


Fig.  10. — The  stomach  with  the  pancreas  and  duodenum:  C  is 
placed  at  the  cardiac  opening  of  the  stomach,  while  P  is  at  the 
pylorus.  A  dotted  Hne  is  used  to  complete  the  form  of  the  pancreas, 
which  discharges  to  the  intestine  near  W.  The  shape  of  the  stomach 
is  of  one  moderately  filled;  with  further  distention  the  lower  border 
of  the  organ  would  sag  and  the  pylorus  would  cease  to  be  the  lowest 
point.  B  is  the  common  bile-duct,  which  reaches  the  intestine  be- 
hind at  the  same  point  at  which  the  pancreas  delivers  its  secretion. 

called  the  colon.  Beginning  with  a  small  rounded  pouch, 
the  cecum,  it  may  be  followed  upward  on  the  right  side  of 
the  body  to  the  level  of  the  lower  ribs.  This  part  is  the 
ascending  colon.  From  here  it  bends  sharply  to  the  left 
and  crosses  the  full  width  of  the  abdominal  cavity.  This 
horizontal  segment  is  known  as  the  transverse  colon  and 
lies  close  to  the  ventral  body  wall.  Thus  it  passes  in  front 
of  the  duodenum  and  is  in  practical  contact  with  the 


THE    ALIMENTARY   CANAL  59 

stomach.  At  the  left  side  of  the  body  and  near  the  spleen 
the  descending  colon  begins.  Its  course  is  downward 
and  backward,  so  that  it  passes  behind  the  coils  of  the 
small  intestine.  Following  the  dorsal  boundary  of  the 
cavity  around  to  the  middle  line,  the  colon  forms  the  short, 
curved  region  called  the  sigmoid  flexure.  The  remaining 
section  is  the  rectum,  situated  directly  in  front  of  the  lower 
extremity  of  the  spinal  column  within  the  pelvis  and  ter- 
minating at  the  anus. 

Almost  everywhere  the  lining  of  the  alimentary  canal  is 
pitted  with  microscopic  glands.  Those  in  the  stomach 
furnish  the  gastric  juice;  those  in  the  intestine,  the  intesti- 
nal juice.  Besides  these  small  glands  and  the  salivary 
glands  already  mentioned,  there  are  the  pancreas  and  the 
liver,  contributing  secretions  to  the  cavity  of  the  digestive 
tract.  The  pancreas  has  been  said  to  lie  in  the  turn  of  the 
duodenum.  It  is  thus  under  the  pyloric  portion  of  the 
stomach  and  behind  the  transverse  colon.  Its  main  duct 
discharges  into  the  small  intestine  about  3  inches  below  the 
pylorus.  A  second,  but  very  small,  duct  opens  close  by. 
The  liver,  which  is  the  largest  gland  in  the  body,  is  fitted 
to  the  concave  under  surface  of  the  diaphragm  and  is 
mainly  to  the  right  of  the  midplane.  It  is  cleft  into  several 
lobes,  from  which  ducts  converge  and  unite  as  they  ap- 
proach the  duodenum.  A  single  duct  is  finally  formed  and 
it  enters  the  intestine  at  the  same  point  as  the  chief  pan- 
creatic duct.  The  arrangement  serves  to  blend  the  two 
secretions,  and  is  somewhat  suggestive  of  the  devices  used 
with  bath-tubs  for  mingling  hot  and  cold  water. 

The  liver  produces  bile,  and  its  channel  of  discharge  to 
the  duodenum  is  accordingly  known  as  the  bile-duct. 
This  duct  has  a  side  branch  which  leads  to  a  contractile 
sac  embedded  in  the  under  surface  of  the  liver,  this  reser- 
voir being  the  gall-bladder.  Bile,  as  it  flows  down  from 
the  liver,  may  either  find  its  way  directly  to  the  intestine 
or  it  may  turn  aside  into  the  gall-bladder.  The  course 
taken  will  depend  on  the  contraction  and  relaxation  of  the 
muscular  walls  of  the  ducts.     Active  contraction  of  the 


60  NUTRITIONAL    PHYSIOLOGY 

gall-bladder  when  it  is  full  may  send  a  considerable  amount 
of  bile  at  one  time  into  the  intestine.  The  relation  of  the 
gall-bladder  to  the  liver  is  like  that  of  the  urinary  bladder 
to  the  kidneys,  at  least  to  the  extent  that  its  existence 
makes  possible  a  continuous  production  of  the  secretion 
with  an  intermittent  emptying.  It  has  been  shown,  how- 
ever, that  the  bile  is  concentrated  and  otherwise  modified 


Fig.  11. — This  is  an  entirely  schematic  section  across  the  human 
body  in  the  mid-abdominal  region:  S  indicates  the  spine;  K,  the 
kidneys;  P  is  the  peritoneum,  the  lining  of  the  abdominal  wall. 
It  is  prolonged  from  the  back  to  form  the  mesentery  (M),  which 
extends  to  and  around  the  loop  of  intestine  (/).  The  large  unoc- 
cupied space  shown  does  not  really  exist,  for  successive  portions  of 
the  alimentary  canal  together  with  other  organs  completely  fill  the 
cavity. 

during  its  stay  in  the  gall-bladder.     The  urine  does  not 
change  its  character  distinctly  while  it  is  in  storage. 

When  the  abdominal  wall  of  an  animal  is  cut  through 
and  laid  back  from  the  organs  within,  one's  first  impression 
is  that  the  viscera  are  lying  unattached  in  the  cavity. 
They  are,  in  fact,  not  adherent  to  the  ventral  or  lateral 
portions  of  the  wall.  But  if  we  take  a  loop  of  the  small 
intestine  at  random  and  attempt  to  lift  it  from  its  resting- 
place,  we  find  it  attached  to  the  middle  of  the  back  by  a 
tough,    transparent   membrane,    the   mesentery.     In   the 


THE   ALIMENTARY    CANAL  61 

mesentery  can  be  seen  blood-vessels,  lymphatics,  and 
nerves.  This  suspending  sheet  thus  serves  not  merely  for 
mechanical  support,  but  also  establishes  connection  be- 
tween the  intestine  and  the  circulatory  and  nervous  sys- 
tems. The  student  is  apt  to  find  it  hard  to  visualize  the 
mesentery  in  its  actual  form ;  he  is  to  imagine  a  membrane 
which  at  one  edge  extends  to  the  entire  length  of  the  small 
intestine  and  to  much  of  the  large,  while  its  other  edge  is 
condensed  to  be  inserted  into  the  space  of  a  few  inches 
before  the  spinal  column.  What  results  from  these  condi- 
tions has  been  described  as  "a,  ruffle  or  flounce,"  Al- 
though the  mesentery  is  thin  it  is  really  a  doubled  sheet 
enveloping  the  intestine.  This  will  be  made  clear  by  the 
diagram,  which  also  shows  how  the  mesentery  is  continu- 
ous with  the  exquisitely  smooth,  lustrous  lining  of  the  ab- 
dominal cavity,  to  which  is  given  the  name  of  peritoneum. 
Dissection  of  a  small  animal  will  give  a  comprehension  of 
these  anatomic  facts  which  can  scarcely  be  gained  by 
reading. 

The  stomach  has  a  supporting  membrane  attached  to  it 
along  its  lesser  curvature  and  uniting  it  to  the  liver,  which 
is,  in  turn,  anchored  to  the  dorsal  body  wall.  This  mem- 
brane is,  in  effect,  a  mesentery  for  the  stomach,  but  is  called 
the  lesser  omentum.  An  extension  of  similar  tissue  hangs 
from  the  greater  curvature  like  an  apron  over  the  intestinal 
coils  and  is  called  the  great  omentum.  It  may  become 
a  ponderous  appendage  from  the  fat  which  it  sometimes 
accumulates.  The  continuation  of  the  mesentery  over 
the  external  surface  of  the  intestine  and  the  identical 
covering  of  the  stomach  form  for  these  organs  what  is 
spoken  of  as  their  serous  coat. 

The  Finer  Structure  of  the  Alimentary  Organs. — We 
have  said  that  the  internal  surface  of  the  stomach  and  of 
both  intestines  is  provided  with  glands.  The  inner  layer 
of  the  wall  of  the  canal  in  which  these  glands  occur  is 
called  the  mucous  coat  or  mucous  membrane.  This  is  in 
reference  to  the  fact  that  its  exposed  cells  produce  the  slimy 
substance,  mucus,  more  familiarly  associated  with  nasal 


62  NUTRITIONAL   PHYSIOLOGY 

discharges.  It  probably  acts  as  a  lubricant  in  the  digestive 
tract  and  also  protects  the  lining  cells  from  harsh  contacts, 
both  physical  and  chemical.  The  mucous  membrane  is 
depressed  to  form  the  recesses  of  the  glands,  and  in  the 
small  intestine  is  raised  into  the  microscopic  prominences 
referred  to  as  villi.  Underlying  it  blood-vessels  and  nerve- 
fibers  run  thickly. 

Between  the  mucous  layer  and  the  serous  coat  on  the 
outside  of  the  intestine  there  is  a  development  of  muscle 
of  the  order  described  in  a  previous  chapter  as  characteristic 
of  most  internal  organs,  that  is  to  say,  slow  acting,  more  or 
less  automatic,  and  much  given  to  tone  changes.  In  the 
small  intestine  there  are  two  distinct  muscular  coats:  the 
inner  and  thicker  has  its  fibers  at  right  angles  to  the  axis 
of  the  canal,  while  the  outer  has  them  set  parallel  to  this 
axis.  The  inner  coat  is  hence  spoken  of  as  circular  and  the 
outer  as  longitudinal.  There  is  no  doubt  of  the  superior 
prominence  of  the  circular  coat  in  the  production  of  intes- 
tinal movements.  In  the  stomach  the  muscular  organiza- 
tion is  less  simple  and  there  are  oblique  elements  in  addi- 
tion to  those  which  can  be  classed  as  circular  and  longi- 
tudinal. The  colon  has  the  circular  coat,  but  instead  of  a 
complete  covering  of  longitudinal  muscle  it  has  three 
bands  of  contractile  tissue  extending  along  its  wall. 

At  any  point  along  the  course  of  the  intestine  temporary 
closure  may  be  effected  by  the  contraction  of  the  circular 
muscle.  But  there  are  certain  places  where  such  closure 
is  far  more  frequent  or,  indeed,  the  usual  condition. 
Where  the  esophagus  joins  the  stomach  an  irritable  band, 
the  cardiac  sphincter,  is  much  of  the  time  firmly  contracted. 
There  is,  similarly,  a  pyloric  sphincter  guarding  the  open- 
ing between  the  stomach  and  the  duodenum.  Where 
the  ileum  enters  the  cecum  a  valve  exists  which  is  adapted 
to  prevent  the  reflux  of  material  from  the  colon  to  the  small 
intestine.  This,  the  ileocecal  valve,  is  probably  reinforced 
in  its  mechanical  action  by  muscular  support.  Finally, 
the  short  anal  canal  is  closed  by  an  inner  sphincter  which 
is  essentially  a  thickening  of  its  own  wall,  and  an  external 
one  composed  of  skeletal  muscle. 


CHAPTER  VII 

THE   MOUTH— SWALLOWING;    SALIVARY 
DIGESTION 

Mastication. — The  hygienic  importance  of  thorough 
mastication  is  undoubted,  but  there  is  little  occasion  for 
any  extended  analysis  of  a  process  so  obvious.  It  is  to  be 
observed  that  the  lower  jaw  does  not  have  merely  an  up- 
and-down  movement,  but  that  it  glides  backward  and 
forward  and  has  some  lateral  play  at  the  same  time.  The 
teeth,  therefore,  do  not  simply  chop  the  food,  but  rub  and 
grind  it.  In  the  work  of  mechanical  reduction  a  larger 
part  is  borne  by  the  tongue  than  is  commonly  recognized. 
The  little  member  seems  to  be  everywhere  at  once,  thrust- 
ing food  between  the  teeth,  withdrawing  it  again,  bruising 
and  rasping  it  against  the  roof  of  the  mouth.  While  this 
action  is  going  on  an  intimate  mixture  with  the  saliva 
is  accomplished.  We  must  now  proceed  to  a  discussion 
of  this  the  first  of  the  digestive  secretions. 

Mention  has  been  made  of  the  three  pairs  of  glands  which 
supply  the  saliva.  Their  united  product  is  estimated  to 
reach  an  amount  of  about  3  pints  a  day,  equalling  the  vol- 
ume of  the  urine.  If  one  finds  it  hard  to  credit  such  a 
statement,  attention  may  be  called  to  the  copious  character 
of  the  flow  which  is  noted  when  one  is  interrupted  at  the 
moment  of  taking  food.  There  is  little  secretion  apart 
from  eating  unless  it  is  excited  by  chewing  sundry  things. 
At  mealtime  a  large  part  of  what  is  swallowed  is  saliva, 
and  the  proportion  must  be  greatly  raised  by  the  practice 
of  prolonged  mastication,  so-called  Fletcherism.  The 
formation  of  saliva  is  to  be  regarded  as  a  reflex  in  which 
the  primary  stimulation  is  furnished  by  food  in  the  mouth 

63 


64  NUTRITIONAL   PHYSIOLOGY 

acting  upon  the  endings  of  nerves  excitable  by  its  chemical 
ingredients  and  by  its  temperature  more  than  by  its  mere 
contact.  We  have  to  do  with  something  more  than 
the  t5rpical' reflex,  however,  because  it  is  a  familiar  fact 
that  the  appeal  to  consciousness  has  much  influence  upon 
the  flow  of  saHva.  The  "watering  of  the  mouth"  at  the 
approach  of  acceptable  food  is  a  hard  phenomenon  to 
classify.  It  is  a  reflex,  but  it  is  one  which  would  not 
occur  in  an  unconscious  subject.  For  such  actions  the 
term  psychoreflex  is  often  used. 

The  sahva  is  a  bland  fluid  which  one  would  hardly  sup- 
pose to  be  endowed  with  active  powers  of  digestion.  In 
some  animals  it  does  not  have  any  apparent  chemical  ac- 
tion. Still,  it  has  valuable  properties  which  we  shall  do 
well  to  recognize.  Whether  it  is  a  digestive  juice  or  not, 
its  physical  effect  is  useful  in  mastication,  since  it  softens 
the  food,  makes  it  cohere  into  the  pellets  which  are  pre- 
pared for  swallowing,  and  later  lubricates  their  transit  to 
the  stomach.  Moreover,  it  has  a  defensive  use,  protecting 
the  mouth  from  injury  when  food  or  fluid  is  taken  too  hot 
or  when  some  corrosive  liquid  calls  for  dilution.  As  it  issues 
fresh  from  the  glands  it  is  slightly  alkaline  in  reaction. 
If  it  stagnates  for  a  long  time  in  the  by-places  of  the  mouth, 
as  happens  during  sleep,  and  if  it  contains  at  the  same  time 
traces  of  carbohydrate  food  in  solution,  bacterial  fermenta- 
tion may  make  it  acid  and  the  effect  upoii  the  teeth  may 
be  injurious.  The  value  of  an  alkaline  mouth- wash,  like 
milk  of  magnesia,  used  at  bedtime  is  evident. 

Human  saliva  contains  various  salts.  Attention  need 
be  called  only  to  its  lime  compounds,  which  are  always  de- 
posited more  or  less  upon  the  back  surfaces  of  the  teeth, 
a  process  which  reminds  one  of  the  formation  of  stalactites 
and  stalagmites  in  caverns.  The  hard  crust  that  results 
is  the  tartar.  It  is  not  very  unlike  the  original  substance 
of  the  teeth  in  its  chemical  composition,  and  its  occurrence 
might  seem  to  indicate  a  mode  of  making  good  the  wearing 
away  of  the  teeth.  Unfortunately  we  cannot  regard  it  in 
this  favorable  light,  for  the  lime  salts  are  always  contami- 


THE    MOUTH — SWALLOWING;    SALIVARY    DIGESTION      65 

nated  with  food  particles  and  bacteria.  The  deposit 
should  be  removed  by  the  dentist  at  regular  intervals. 

The  three  glands  furnish  slightly  different  varieties  of 
saliva.  Mucin,  the  essential  compound  in  mucus,  is  pres- 
ent in  the  secretion  of  the  two  lower  glands  and  not  in 
that  of  the  parotid.  It  gives  to  the  saliva  from  the  sub- 
maxillary and  sublingual  glands  a  ropy,  mucilaginous 
character,  which,  of  course,  becomes  more  apparent  when 
evaporation  has  concentrated  the  solution.  This  is  illus- 
trated when  the  mouth  is  dried  by  rapid  breathing  during 
exercise  and  becomes  furred  with  the  residue  of  salivary 
mucin.  This  constituent  of  the  saliva  probably  makes  it 
superior  to  water  as  an  agent  for  molding  the  food  into 
pellets.  The  most  interesting  property  of  the  secretion, 
its  power  to  hydrolyze  starch,  may  be  discussed  to  more 
advantage  after  we  have  followed  the  food  to  the  stomach. 
Clearly,  there  is  not  time  enough  for  much  digestive  change 
in  the  mouth  of  a  person  of  average  habits. 

Swallowing. — The  transfer  to  the  stomach  is  a  more 
complex  matter  than  is  likely  to  be  realized.  It  involves 
an  interruption  of  breathing  and  the  protection  of  the  nasal 
passages  and  the  larynx  against  the  intrusion  of  food.  The 
first  purpose  is  effected  by  the  swinging  back  of  the  soft 
palate  against  the  back  of  the  phar^mx.  The  second  is 
accomplished  by  the  drawing  forward  of  the  larynx  toward 
the  chin,  a  movement  which  can  be  plainly  felt.  By  it 
the  larynx  is  tucked  under  the  root  of  the  tongue  and  over- 
laid by  the  epiglottis.  The  same  motion  serves  to  widen 
the  upper  part  of  the  esophagus,  which  is  not  usually  ap- 
preciably open.  With  the  parts  in  this  position  the  bolus 
of  food  is  crowded  back  from  its  original  seat  upon  the 
tongue  and  urged  through  the  pharynx  by  the  successive 
contraction  of  the  bands  of  muscle  which  surround  it. 
As  soon  as  it  is  fairly  within  the  esophagus  the  soft  palate 
is  lowered,  the  larjmx  is  allowed  to  emerge  from  its  covert, 
and  the  breathing  can  be  resumed.  Such  quick  and  well- 
ordered  adjustments  give  evidence  of  co-ordinated  reflex 
action,  the  contact  of  the  food  morsel  with  one  spot  after 
5 


66 


NUTRITIONAL   PHYSIOLOGY 


another  furnishing  the  requisite  stimuli.  We  cannot  go 
through  the  series  of  movements  unless  there  is  at  least 
a  little  saliva  to  be  swallowed,  and  we  cannot  arrest  the 
march  of  events  when  it  is  once  begun. 

The  contractile  tissue  in  the  upper  part  of  the  esophagus 
is  of  a  skeletal  variety.  Lower  down  this  gives  place  to 
typical  visceral  muscle.  Hence  it  is  not  strange  to  find  that 
the  advance  of  the  bolus  becomes  progressively  slower  as 
it  descends.     The  movement  which  is  here  taking  place 


Fig.  12. — An  exaggerated  representation  of  peristalsis.  I  and  II 
are  successive  views  of  the  same  portion  of  the  alimentary  tube: 
P  is  the  zone  of  contraction  shifting  downward  and  always  pre- 
ceded by  the  zone  of  unusual  relaxation  (A'').  Ill  is  an  imaginary 
section  through  II,  showing  the  food  bolus  (6)  slipping  along  in 
advance  of  the  contracting  region,  its  advance  being  facilitated 
by  the  relaxation  below. 

is  what  is  known  as  a  peristalsis,  and  it  is  highly  important 
that  its  principle  should  be  understood.  The  most  ob- 
vious feature  is  a  ring  of  contraction  setting  in  above  the 
enclosed  pellet,  causing  it  to  slide  onward,  and  following 
it  down  by  involving  in  succession  each  level  of  the  tube. 
The  mechanical  application  can  be  simply  illustrated  by 
propelling  a  glass  bead  through  a  soft-rubber  tube  by 
pinching  repeatedly  behind  it  with  thumb  and  finger.  A 
strict  analysis  of  what  occurs  in  the  esophagus  obliges  us  to 
recognize  that  the  process  is  not  so  simple  as  it  first  appears. 


THE    MOUTH — SWALLOWING;    SALIVARY   DIGESTION      67 

There  seem  to  be  two  phases  in  what  is  called  the  peristal- 
tic wave.  The  eye  detects  chiefly  the  traveling  contrac- 
tion, but  this  is  apparently  preceded  by  a  zone  of  unusual 
relaxation,  a  region  of  inhibition. 

The  peristaltic  wave  which  is  necessary  for  the  propul- 
sion of  solid  food  does  not  seem  to  be  required  to  send  liquid 
to  the  stomach.  A  swallow  of  water  is  shot  swiftly  from 
the  mouth  to  the  cardiac  sphincter  and  arrives  there  dis- 
tinctly in  advance  of  the  plodding  peristalsis.  When  one 
drinks  a  glass  of  water,  the  swallows  following  in  rapid  suc- 
cession, a  single  peristaltic  wave  ends  the  series.  Of 
course,  when  fluid  is  carried  up-grade  in  the  esophagus,  as 
when  a  horse  is  drinking  from  a  pool  at  his  feet,  active 
peristalsis  is  as  necessary  as  though  solid  food  were  being 
moved,  and  one  may  plainly  see  the  passing  of  each  swallow 
along  the  extended  neck.  We  shall  find  that  the  small 
intestine  exhibits  movements  which  are  approximately 
the  same  in  principle  as  those  of  the  esophagus,  but  far 
slower  and  usually  less  energetic. 

Salivary  Digestion. — Within  the  stomach  the  accumu- 
lated food  with  a  large  admixture  of  saliva  lies  for  some  time 
with  little  motion.  Here  then  salivary  digestion  must  take 
place.  The  statement  has  been  made  that  in  some  animals 
the  saliva  has  only  mechanical  and  protective  functions. 
More  frequently,  however,  it  has  the  power  to  hydrolyze 
starch,  forming  malt-sugar  as  the  chief  end-product.  This 
seems  to  justify  the  assumption  that  an  enzyme  is  present, 
and  it  is  variously  named  ptyalin,  salivary  amylase,  or 
salivary  diastase.  Such  an  enzyme  probably  plays  an  im- 
portant part  in  the  digestive  processes  of  ruminants,  ani- 
mals which  chew  the  cud.  Human  saliva  acts  upon  starch 
with  surprising  energy.  A  simple  demonstration  of  the 
fact  may  be  had  by  holding  a  bread-crumb  in  the  mouth 
longer  than  is  habitual,  when  it  will  gradually  develop  a 
mildly  sweet  taste. 

The  prevailing  opinion  in  regard  to  the  amount  of  diges- 
tion accomplished  by  the  saliva  in  man  has  undergone  a 
change  during  the  last  few  years.     It  is  allowed  a  larger 


68  NUTRITIONAL   PHYSIOLOGY 

place  than  was  formerly  granted  to  it.  The  enzyme  is 
extremely  sensitive  to  acid.  Inasmuch  as  the  gastric  juice 
is  decidedly  acid,  it  used  to  be  claimed  that  salivary  diges- 
tion could  not  proceed  in  the  stomach.  But  it  has  come 
to  be  recognized  that  when  a  large  mass  of  food  is  intro- 
duced into  the  stomach  within  a  short  time  the  gastric  juice 
penetrates  it  rather  slowly.  A  few  minutes  after  the  com- 
pletion of  a  meal  we  may  picture  the  stomach-contents  as 
being  acidified  near  the  surface,  the  acid  slowly  making  its 
way  inward,  but  having  a  neutral  or  even  alkaline  central 
portion.  Salivary  digestion  will  be  continued  in  the  stead- 
ily diminishing  region  not  yet  reached  by  the  acid,  and  will 
cease  only  when  the  gastric  secretion  from  one  wall  of  the 
stomach  meets  that  from  the  other.  Any  rotation  of  the 
contents  would  probably  bring  about  an  earlier  distribution 
of  the  acid  and  arrest  of  starch  digestion.  No  such  rota- 
tion seems  normally  to  occur.  A  factor  which  operates  to 
postpone  the  destruction  of  ptyalin  is  the  power  of  the 
proteins  of  the  diet  to  engage  hydrochloric  acid  in  com- 
bination. Since  proteins  are  almost  always  present,  the 
gastric  glands  must  secrete  acid  enough  to  satisfy  their 
capacity  before  there  can  be  the  excess  of  strictly  free  acid 
which  will  put  an  end  to  salivary  digestion. 

If  the  mixed  food  is  quite  acid  at  the  outset,  it  is  hard  to 
see  how  there  can  be  any  hydrolysis  of  starch  brought 
about  by  the  saliva.  Yet  we  constantly  eat  acid  fruits 
before  our  breakfast  cereal  and  notice  no  ill  effects. 
Starch  which  escapes  digestion  at  this  stage  is  destined  to 
be  acted  upon  by  the  pancreatic  juice,  and  the  final  result 
may  be  entirely  satisfactory.  Still  it  is  reasonable  to  as- 
sume that  the  greater  the  work  done  by  the  saliva,  the 
lighter  will  be  the  task  remaining  for  the  other  secretions 
and  the  greater  the  probability  of  its  complete  accomplish- 
ment. The  power  of  saliva  to  convert  raw  starch  to  sugar 
is  almost  incomparably  smaller  than  its  capacity  to  digest 
starch  which  has  been  cooked.  Raw  starch  exists  in  very 
dense  grains  which  have  to  be  dissolved  from  the  surface 
inward.     Cooking,  especially  boihng,  utterly  destroys  these 


THE   MOUTH — SWALLOWING;    SALIVARY    DIGESTION      69 

grains,  and  permits  a  reaction  between  the  enzyme  and 
the  separated  molecules  of  the  carbohydrate. 

The  change  from  starch  to  sugar  seems  not  to  be  effected 
by  a  single  reaction,  but  by  stages.  Physiologic  chemists 
have  studied  extensively  the  numerous  intermediate  bodies 
which  have  a  fugitive  existence  in  the  process.  Most  of 
these  are  covered  by  the  term  dextrins.  It  is  sufficient  for 
our  present  purpose  to  regard  them  as  carbohydrates, 
simpler  in  their  molecular  structure  than  the  original  starch, 
but  complex  as  compared  with  the  familiar  sugars.  We 
have  said  that  the  chief  product  of  salivary  hydrolysis  is 
malt-sugar  or  maltose.  This  is  one  of  several  sugars 
classed  as  disaccharids.  It  can  be  hydrolyzed  further  to 
form  dextrose  (or  glucose),  a  sugar  of  the  simplest  type, 
and  one  which  is  ready  to  be  absorbed  and  to  minister 
to  the  living  tissues.  Some  dextrose  is  said  to  be  formed 
in  prolonged  salivary  digestion,  but  the  cleavage  lags 
when  the  maltose  stage  has  been  reached. 


CHAPTER  VIII 
THE  MOVEMENTS   OF  THE   STOMACH 

It  will  be  recalled  (Chapter  VI)  that  the  stomach  consists 
of  a  main  rounded  portion  from  which  a  much  smaller 
conical  segment  extends  to  the  right  to  join  the  duodenum 
at  the  pylorus.  The  larger  part  is  called  the  fundus,  the 
tapering  region  is  the  antrum.  The  muscular  coats  of  the 
antrum  are  somewhat  thicker  than  those  of  the  fundus  and 
show  an  especially  conspicuous  development  of  the  circu- 
lar elements.  There  is  no  such  contrast  between  the  two 
parts  of  the  human  stomach  as  between  the  thin-walled 
crop  and  massive  gizzard  of  the  bird,  but  there  is  a  faint 
suggestion  of  an  analogous  difference.  The  fundus  is, 
indeed,  primarily  a  place  for  the  storage  of  food;  the  an- 
trum, while  not  a  crushing  mechanism,  has  distinctly 
greater  motor  properties. 

The  antrum  is  considered  to  be  set  off  from  the  fun- 
dus by  the  so-called  transverse  hand.  This  is  an  irritable 
ring  of  the  circular  muscle  which  is  often  contracted  enough 
to  indent  the  outline  of  the  stomach  at  this  point,  and  which 
may  occasionally  create  a  temporary  division  of  the  gas- 
tric cavity  into  two  parts.  It  has  been  called  the  sphinc- 
ter of  the  antrum,  but  it  cannot  fairly  be  compared  with 
the  cardiac  and  the  pyloric  sphincters,  since  these  are 
habitually  closed,  while  closure  at  the  transverse  band  is 
rare. 

Regulation  of  the  Cardiac  Sphincter. — Food  and  drink 
entering  the  stomach  pass  the  cardiac  sphincter.  The 
guardian  muscle  is  usually  more  or  less  contracted.  It 
relaxes  upon  the  arrival  of  the  peristaltic  wave  in  the 
esophagus.  If  it  is  recalled  that  the  peristalsis  consists  of 
a  wave  of  inhibition  running  before  a  contraction,  it  is 

70 


Fig.  13. — To  suggest  the  probable  appearance  of  the  distended 
and  active  human  stomach.  Three  marked  waves  of  contraction 
are  seen  in  the  antral  region.  These  are  to  be  conceived  gf  as  pass- 
ing onward  toward  the  pjdorus. 


Ifei. 


'>, 


V  ^^-^ 


THE   MOVEMENTS    OF   THE    STOMACH  71 

easy  to  see  how  the  cardia  may  be  opened  at  the  moment 
when  its  muscular  walls  fall  under  the  influence  of  the 
phase  of  relaxation.  Closure  will  follow  immediately,  as 
the  second  or  positive  part  of  the  peristalsis  involves  the 
sphincter.  Attention  has  been  called  to  the  fact  that  liq- 
uids may  outrun  a  pursuing  peristalsis  and  arrive  several 
seconds  in  advance  of  it  at  the  cardiac  opening.  Under 
such  circumstances  it  is  said  that  the  fluid  remains  at 
the  bottom  of  the  esophagus  until  overtaken  by  the  wave, 
when  the  relaxation  occurs  which  permits  it  to  pass  into 
the  stomach.  A  person  drinking  with  ill-advised  haste 
may  have  the  disagreeable  experience  of  filling  the  esoph- 
agus enough  to  produce  a  painful  distention.  Relief 
comes  abruptly  when  the  peristalsis  has  made  its  way  to 
the  cardiac  sphincter  and  secured  an  entrance  for  the 
liquid. 

While  this  has  been  the  generally  accepted  description 
of  the  facts,  it  has  been  shown  recently  that  the  cardia  is 
not  always  so  firmly  contracted.  Observations  by  a;-ray 
methods,  to  be  described  presently,  have  shown  that  for 
some  time  after  a  meal  there  may  be  a  reflux  from  the 
stomach  of  a  cat  into  the  esophagus.  Each  escape  of  food 
evokes  a  local  peristaltic  wave  which  returns  it  to  the 
stomach.  Such  an  incident  does  not  entail  any  movement 
of  the  throat  muscles  and  is  probably  subconscious.  As 
the  period  of  digestion  continues  the  sphincter  becomes 
more  tightly  set  and  no  longer  allows  any  such  return  of 
stomach-contents.  The  increased  tension  has  a  simple 
explanation,  which,  like  many  another  point  about  the 
stomach,  we  owe  toW.  B.  Cannon.  He  has  shown  that 
the  tension  is  developed  in  response  to  the  rise  of  acidity 
in  the  liquid  just  within  the  cardia.  Since  the  acid  appears 
normally  after  each  filling  of  the  stomach,  we  have  here 
an  automatic  provision  for  the  establishment  of  the  requi- 
site guard  over  this  opening.  The  influence  of  the  ner- 
vous system  seems  capable  of  nullifying  the  local  effect  of 
acid,  since  the  sphincter  may  be  relaxed  to  permit  vomit- 
ing at  times  when  the  gastric  contents  are  excessively  acid. 


72  NUTRITIONAL   PHYSIOLOGY 

The  Fundus. — An  important  service  of  the  stomach  is 
to  store  food  in  relatively  large  quantities  at  mealtimes 
and  to  deliver  it  gradually  to  the  intestine.  A  person  who 
has  been  deprived  of  the  stomach,  or  of  most  of  it,  by  sur- 
gery is  made  aware  of  this  when  he  finds  it  impossible  to 
eat  "a  square  meal,"  and  is  compelled  to  take  small  por- 
tions of  food  at  short  intervals.  He  is  then  serving  his 
intestines  somewhat  as  they  are  normally  treated  by  the 
stomach.  Storage  is  not  the  sole  function  of  the  stomach, 
but  we  do  well  to  emphasize  it.  The  fundus  accommodates 
itself  to  its  contents  by  tone  changes,  relaxing  when  food 
is  being  swallowed  and  afterward  exerting  a  steady,  mod- 
erate pressure  which  insures  the  filling  of  the  antrum  after 
every  discharge  at  the  pylorus.  A  lack  of  this  tonic  re- 
action may  be  a  cause  of  serious  disorders. 

The  earlier  writers  often  claimed  that  there  is  a  definite 
and  regular  overturning  of  the  contents  of  the  fundus. 
In  the  light  of  more  recent  observations  this  does  not  seem 
to  be  usual.  A  German  investigator  fed  to  a  rat  three 
courses  of  food  of  contrasted  colors.  The  animal  was  then 
killed  and  frozen.  A  section  made  through  the  hardened 
mass  within  the  stomach  showed  distinct  stratification. 
The  food  first  taken  was  in  the  antrum  and  the  lower  part 
of  the  fundus,  the  second  instalment  was  above  the  first, 
and  the  third  was  just  under  the  cardia.  It  seems  hardly 
probable  that  entirely  liquid  food  could  remain  thus  strati- 
fied when  one  considers  the  extent  to  which  the  stomach  is 
subjected  to  the  influence  of  bodily  movements. 

oc-RsLj  Studies  of  the  Stomach. — While  much  can  be 
learned  of  the  behavior  of  the  stomach  through  experi- 
ments involving  its  exposure  by  surgical  procedures,  the 
ideal  method  is  clearly  one  which  leaves  the  animal  in  its 
normal  condition.  Such  a  method  became  available  when 
the  x-ray  was  first  turned  to  account  to  observe  visceral 
movements.  The  image  of  any  part  of  the  body  projected 
by  means  of  the  x-ray  shows  the  bones  in  clear  contrast 
with  the  softer  parts,  but  scarcely  outhnes  the  organs.  If, 
however,  any  harmless  substance  opaque  to  the  x-ray  is 


THE    MOVEMENTS    OF   THE    STOMACH  73 

introduced  into  the  contents  of  the  alimentary  canal,  it 
becomes  possible  to  recognize  the  situation  of  this  sub- 
stance so  long  as  it  remains  sufficiently  concentrated. 
More  than  this,  if  the  cavity  is  well  filled  its  outline  is,  of 
course,  identical  with  that  of  the  included  material.  The 
a;-ray  picture  will  then  show  the  changing  contour  of  the 
organ  in  silhouette.  The  compounds  most  used  to  secure 
opacity  to  the  x-ray  are  the  salts  of  bismuth,  generally  the 
subnitrate  or  the  subcarbonate. 

The  most  numerous  experiments  of  this  sort  are  those  of 
Cannon,  and  the  cat  has  been  the  favorite  subject.  When 
the  animal  has  had  a  full  meal  of  bread-crumbs  and  milk 
with  the  bismuth  salt  evenly  mixed  in  the  mass  the  x-ray 
shows  the  entire  form  of  the  stomach.  The  fundus  has  an 
even  outline  and  preserves  it  unchanged  from  hour  to 
hour,  except  that  a  very  gradual  contraction  takes  place. 
The  antrum  is  traversed  by  deep  but  slow-moving  peris- 
taltic waves,  which  originate  near  the  transverse  band  and 
pass  to  the  pylorus.  The  tendency  of  such  waves  must  be 
to  force  successive  portions  of  the  food  into  the  intestine, 
but  in  the  great  majority  of  cases  the  waves  bear  down 
upon  a  tightly  closed  pyloric  sphincter.  The  only  possible 
result  is  then  an  eddying  movement,  the  contents  advanc- 
ing only  to  rebound  from  what  is,  for  the  time,  a  blind 
pouch.  This  favors  the  reduction  of  the  larger  morsels  and 
helps  to  secure  at  length  the  formation  of  the  smooth, 
creamy  "chyme."  But  it  is  probable  that  most  people 
have  an  exaggerated  notion  of  the  mechanical  powers  of 
the  stomach. 

The  waves  which  pass  over  the  antrum  arise  in  the  cat 
with  strange  regularity  at  intervals  of  ten  seconds.  Each 
wave  takes  about  half  a  minute  to  make  its  way  to  the 
pylorus,  so  there  are  commonly  three  creases  to  be  seen, 
all  shifting  with  a  motion  of  clock-like  slowness  toward  the 
outlet.  During  the  prolonged  period  required  for  the 
emptying  of  the  stomach  of  the  cat — eight  hours  or  more — 
it  is  evident  that  the  total  number  of  the  waves  may  be 
over  two  thousand.     When  the  pyloric  sphincter  momen- 


74  NUTRITIONAL    PHYSIOLOGY 

tarily  relaxes,  under  influences  to  be  discussed  presently, 
the  peristalsis  of  the  antrum  naturally  drives  more  or  less 
of  its  contents  into  the  duodenum. 

Nervous  Control  of  the  Gastric  Movements. — Muscular 
elements  of  the  order  found  in  the  stomach  have  been 
said  to  have  an  automatic  property.  We  have  insisted, 
however,  that  this  fact  does  not  exclude  the  influence  of  the 
central  nervous  system.  There  is  abundant  evidence  so 
far  as  the  stomach  is  concerned  that  the  musculature  of 
the  organ  is  played  upon  by  efferent  impulses.  If  it  is 
separated  from  the  central  nervous  system  many  of  its 
reactions  'take  place  in  a  nearly  normal  manner,  but  we 
cannot  assume  that  its  adjustments  are  as  well  timed  and 
decisive  as  they  were  before.  Laboratory  trials  show  that 
the  impulses  which  are  sent  to  the  stomach  may  either 
accentuate  or  abate  its  spontaneous  movements.  In 
other  words,  they  may  either  excite  or  inhibit  the  contrac- 
tile elements. 

Of  the  two  types  of  nervous  control,  the  inhibitory 
seems  to  be  of  particular  significance.  Complete  arrest  of 
the  peristalsis  of  the  antrum  may  be  brought  about.  This 
happens  in  the  cat  when  the  animal  is  enraged  or  terrified, 
and,  indeed,  when  it  seems  merely  to  be  restless.  Cannon 
has  again  and  again  seen  the  peristaltic  notches  fading 
away  from  the  o^-ray  profile  of  the  antrum  when  the  animal 
has  wearied  of  being  kept  under  restraint,  and  is  manifest- 
ing its  feeling  by  switching  its  tail  and  struggling.  He  has 
seen  the  regular  activity  resumed  when  the  cat  has  been 
pacified.  Similar  facts  have  been  demonstrated  for  the 
rabbit.  Since  we  generally  believe  that  the  higher  the 
grade  of  an  animal's  development,  the  more  extensive  the 
command  of  the  nervous  system  over  its  organs,  we  have 
every  reason  to  think  that  unpleasant  emotions  may  be 
accompanied  in  man  also  by  inhibition  of  the  gastric  move- 
ments. We  shall  have  occasion  to  enlarge  upon  this  matter 
in  connection  with  the  Hygiene  of  Nutrition. 

The  Pyloric  Sphincter. — The  regulation  of  the  escape 
of  the  stomach-contents  to  the  intestine  has  long  been  a 


THE    MOVEMENTS    OF   THE    STOMACH  75 

subject  of  interest.  Nearly  eighty  years  ago  William 
Beaumont  published  his  observations  upon  the  stomach  of 
Alexis  St.  Martin,  a  young  Canadian  trapper,  who  had 
suffered  a  gunshot  wound  in  the  left  side,  in  consequence 
of  which  he  had  a  permanent  gastric  fistula.  The  impres- 
sion which  went  abroad  from  this  celebrated  case  has 
seemed  to  convey  to  the  less  scientific  writers  the  idea  that 
the  pylorus  has  a  power  almost  akin  to  intelligent  inspec- 
tion whereby  it  permits  the  passage  of  certain  portions  of 
the  chyme  and  refuses  egress  to  other  portions.  This 
notion  recalls  amusingly  the  teaching  of  Van  Helmont, 
in  the  seventeenth  century,  that  the  soul  of  man  resides  in 
the  pylorus.  It  cannot  be  said  that  all  the  conditions 
affecting  the  discharge  from  the  stomach  are  entirely  clear, 
but  much  progress  has  been  made  in  this  direction. 

The  sphincter  is  influenced  both  by  the  physical  consist- 
ency and  by  the  chemical  reaction  of  the  gastric  contents. 
The  contact  of  coarse,  angular  particles  with  the  adjacent 
mucous  membrane  seems  to  reinforce  its  contraction,  so 
that  such  material  tends  to  be  kept  longer  in  the  stomach. 
A  more  important  factor  with  this  sphincter,  as  with  the 
cardiac,  is  the  acidity  of  the  chyme.  It  was  stated  above 
that  when  the  stomach-contents  becomes  distinctly  acid- 
ified, the  tone  of  the  cardiac  sphincter  is  increased.  The 
acid  in  this  case  is  acting  upon  the  lining  below  the  irritable 
ring.  Comparison  of  the  two  sphincters  shows  it  to  be  a 
principle  applicable  to  both — that  acid  acting  immediately 
above  favors  their  relaxation,  while  acid  below  causes  them 
to  tighten.  If  this  is  the  main  factor  in  regulating  the 
pylorus,  the  first  opening  will  occur  when  the  acidity  in  the 
antrum  has  reached  a  certain  point.  The  next  peristaltic 
wave  will  transfer  a  little  chyme  to  the'duodenum.  At  this 
instant  there  will  be  acid  material  both  above  and  below 
the  sphincter.  The  action  from  below  appears  to  predom- 
inate, so  that  closure  will  be  established  and  maintained 
until  the  acid  in  the  duodenum  is  either  neutralized  or  re- 
moved somewhat  from  the  pylorus.  When  the  stimula- 
tion from  below  is  no  longer  effective  the  acid  above  will 


76  NUTRITIONAL   PHYSIOLOGY 

cause  a  second  gaping  of  the  sphincter,  followed  as  before 
by  prompt  and  decisive  contraction.  A  more  efficient 
mechanism  to  insure  gradual  delivery  to  the  intestine 
without  distending  it  locally  can  scarcely  be  conceived. 
When  the  latest  portions  of  a  meal  are  leaving  the  stomach, 
the  first  which  went  out  may  have  reached  the  colon,  and 
intermediate  fractions  may  be  undergoing  digestion  in 
numerous  loops  of  the  intervening  small  intestine. 

Our  meals  are  usually  of  a  mixed  character,  including 
proteins,  fats,  and  carbohydrates.  For  purposes  of  experi- 
ment single  food-stuffs  may  be  fed  to  an  animal  and  the 
rate  of  departure  from  the  stomach  noted  for  each.  The 
a;-ray  has  been  employed  for  this  purpose.  Carbohydrates 
have  a  striking  tendency  to  escape  rapidly  to  the  intestine. 
The  discharge  of  proteins  and  of  fats  is  relatively  much  de- 
layed. We  must  be  content  with  stating  the  fact  without 
undertaking  to  discuss  its  somewhat  complex  causes. 

Vomiting. — The  occasional  expulsion  of  the  stomach- 
contents  through  the  cardia  and  esophagus  is  accomplished 
as  the  result  of  a  reflex  movement  in  which  the  chief  mus- 
cles involved  are  not  the  coats  of  the  stomach,  but  the 
contractile  tissue  of  the  diaphragm  and  abdominal  wall. 
When  these  contract  simultaneously  a  high  pressure  is 
thrown  upon  the  stomach.  Such  a  pressure  may  accom- 
pany the  act  of  straining  or  bracing  the  body  for  lifting, 
but  does  not  ordinarily  result  in  vomiting,  it  would  appear, 
because  of  the  resistance  offered  by  the  cardiac  sphincter. 
In  the  crisis  of  nausea,  however,  convulsive  movements 
of  this  kind  take  place  with  inhibition  of  the  sphincter. 
With  the  passage  open,  each  application  of  intense  pressure 
to  the  stomach  may  drive  a  portion  of  its  contents  to  the 
exterior.  The  palate  meanwhile  has  assumed  the  same 
position  as  for  swallowing ;  the  larynx  is  drawn  forward  and 
is  shielded  by  the  root  of  the  tongue,  which  is  depressed  and 
grooved.  At  every  descent  of  the  diaphragm  the  capacity 
of  the  thorax  is  increased,  and  as  no  air  is  permitted  to 
enter  the  lungs  the  esophagus  is  dilated. 

During  the  act  of  vomiting  the  fundus  is  said  to  contract 


THE    MOVEMENTS    OF   THE    STOMACH  77 

steadily  upon  its  diminishing  contents.  This  is  not  a 
movement  powerful  enough  to  secure  the  emptying  of  the 
stomach,  but  adapts  it  to  be  gripped  effectively  by  the 
muscles  of  the  body  wall.  The  transverse  band  is  at  the 
same  time  strongly  contracted  and  the  antrum  has  a  very 
small  volume.  The  pyloric  sphincter  is  said  to  be  closed, 
but  it  is  a  familiar  fact  that  bile  from  the  duodenum  may 
be  pressed  backward  into  the  stomach  under  the  stress  of 
violent  vomiting.  Profuse  salivation  precedes  and  accom- 
panies the  act.  Such  a  reflex  has  a  manifest  value  when 
it  serves  to  remove  from  the  stomach  material  which  might 
prove  poisonous.  It  occurs,  however,  under  many  circum- 
stances when  it  seems  not  to  have  any  advantageous  re- 
sult. Its  apparent  uselessness  in  seasickness  has  already 
been  alluded  to.  It  appears  equally  illogical  when,  in 
pregnancy,  it  is  excited  by  irritation  of  the  pelvic  nerves. 


CHAPTER  IX 
GASTRIC  SECRETION  AND  DIGESTION 

It  was  in  the  eighteenth  century  that  the  chemical 
factors  in  digestion  were  first  clearly  separated  from  the 
mechanical.  The  accounts  which  have  been  preserved 
of  the  experiments  of  Reaumur  (1752)  and  of  Spallanzani 
(1777)  are  of  extraordinary  interest.  An  entertaining 
summary  is  to  be  found  in  Foster's  'lectures  on  the  His- 
tory of  Physiology/'  Chapter  VIII.  These  ingenious 
investigators  were  the  first  to  show  that  digestive  changes 
may  be  caused  to  take  place  outside  the  body  and  in  the 
absence  of  any  mechanical  process  whatever.  They 
obtained  small  quantities  of  gastric  juice 'from  various 
animals,  mixed  it  with  food  in  flasks  and  test-tubes,  and 
watched  for  signs  of  alteration.  Spallanzani,  in  particular, 
succeeded  in  bringing  about  considerable  solution  of  his 
samples. 

From  that  time  to  the  present  studies  of  digestion  have 
continued.  Where  the  pioneers  were  forced  to  be  content 
with  observing  the  dissolving  of  solid  food,  their  successors 
are  drawing  inferences  regarding  the  transformations  of 
unseen  molecules.  A  deeper  insight  into  the  meanings 
of  digestion  became  possible  as  the  new  science  of  organic 
chemistry  was  swiftly  advanced  by  the  researches  of 
Wohler,  Berzelius,  and  Liebig.  As  was  pointed  out  in 
Chapter  III,  it  is  the  molecular  change  which  is  significant 
and  not  the  physical.  Experiments  in  which  material  is 
introduced  into  the  alimentary  canals  of  animals  and  later 
withdrawn  for  analysis  are  constantly  compared  with 
trials  in  which  the  digestion  takes  place  throughout  in  the 
thermostats  of  the  laboratory. 

The  stomach  is  popularly  supposed  to  have  a  very  large 
share  in  the  total  work  of  digestion.     It  cannot,  however, 

78 


GASTRIC    SECRETION    AND    DIGESTION  79 

be  claimed  that  it  is  indispensable  to  man.  It  forms,  as 
already  indicated,  a  convenient  place  of  deposit  for  food 
and  a  gradual  feeder  of  the  small  intestine,  but  it  is  mean- 
while the  seat  of  preliminary  digestive  changes  which 
greatly  facilitate  the  further  advance  of  the  process. 
Attention  has  been  called  to  the  fact  that  salivary  diges- 
tion is  continued  for  a  time  in  the  almost  stationary  con- 
tents of  the  fundxis.  When  this  is  stopped  by  the  penetra- 
tion of  the  acid  gastric  juice  it  is  superseded  by  a  new  type 
of  digestion  in  which  the  proteins  are  the  food-stuffs  acted 
upon.  The  gastric  hydrolysis  of  proteins  is  generally  re- 
ferred to  as  peptic  digestion.  Before  we  discuss  it  in 
detail  we  must  consider  the  nature  and  the  circumstances 
of  formation  of  the  gastric  juice. 

This  secretion  is  the  product  of  the  numerous,  relatively 
simple  glands  with  which  the  mucous  coat  of  the  stomach  is 
provided.  Beaumont  has  vividly  described  the  appearance 
presented  by  the  lining  of  St.  Martin's  stomach  directly 
after  a  meal.  The  surface,  usually  of  a  pale  gray,  flushed 
deeply,  and  the  gastric  juice  welled  up  in  glistening  beads 
from  the  invisible  mouths  of  the  glands.  Its  manner  of 
breaking  out  resembled  the  rising  of  perspiration  from  the 
pores  of  the  skin.  The  empty  stomach  may  have  a  well- 
marked  film  of  mucus  upon  its  walls,  but  its  active  secre- 
tion is  limpid  and  free  flowing.  The  volume  which  the 
human  stomach  produces  in  twenty-four  hours  is  ap- 
parently very  large;  if  we  have  a  right  to  judge  from  what 
is  known  of  the  dog,  it  may  be  3  or  4  quarts.  A  dog,  being 
carnivorous,  probably  secretes  a  disproportionate  quan- 
tity, and  our  estimate  for  man  should  very  likely  be  re- 
duced. 

The  Acid  of  the  Gastric  Juice. — Repeated  reference  has 
been  made  to  the  acidity  of  the  stomach-contents.  The 
early  investigators  were  surprised  and  puzzled  when  they 
were  forced  to  recognize  that  the  acid  in  question  is  largely 
free  hydrochloric.  When  we  consider  that  this  acid  can- 
not be  made  industrially  except  by  decomposing  a  chlorid 
with  the  still  stronger  and  more  corrosive  sulphuric  acid 


80  NUTRITIONAL    PHYSIOLOGY 

and  in  an  earthen  container,  we  can  appreciate  their 
feehng.  For  here  is  what  we  call  a  strong  mineral  acid 
proceeding  from  delicate  living  cells  and  from  fluids  of 
neutral  reaction.  The  formation  of  hydrochloric  acid  by 
the  cells  of  the  gastric  glands  has  become  much  more 
intelligible  in  the  light  of  modern  chemical  teachings. 
The  current  theories  cannot  be  presented  here.  It  is 
evident  that  when  the  elements  of  an  acid  are  withdrawn 
from  a  neutral  fluid  it  must,  theoretically  at  least,  be 
rendered  alkaline.  We  have  a  capital  illustration  of  the 
refinement  of  the  mechanism  by  which  the  body  preserves 
uniform  internal  conditions  in  the  fact  that  when  gastric 
juice  is  being  secreted,  the  urine,  usually  acid  to  common 
indicators,  becomes  alkaline.  Thus  the  normal  chemical 
equilibrium  of  the  blood  is  maintained  sacred  from  disturb- 
ance. The  juice  secreted  into  the  antrum  is  said  not  to  be 
acid. 

Mention  has  been  made  of  some  ways  in  which  the  acid 
of  the  stomach  modifies  local  conditions.  We  have  seen 
that  it  gradually  checks  salivary  digestion.  It  is  the  most 
important  controlling  factor  for  the  two  sphincters. 
Other  features  of  its  action  must  now  be  presented. 
Among  these  is  its  distinct  antiseptic  infleunce.  Spallan- 
zani  noticed  that  pieces  of  meat  undergoing  digestion  in 
gastric  juice  passed  into  solution  without  putrefjang. 
Similar  pieces  kept  for  an  equal  time  in  water  had  radically 
spoiled.  This  was  a  significant  observation  at  the  time, 
because  digestion  and  putrefaction  had  been  regarded  by 
many  as  identical.  We  know  now  that  putrefactive  de- 
composition of  protein  is  due  to  the  influence  of  swarming 
micro-organisms,  and  that  hydrochloric  acid  in  the  con- 
centration usual  in  the  gastric  juice  restrains  the  develop- 
ment of  such  forms. 

The  average  strength  of  the  acid  in  man  is  given  as 
0.2  to  0.3  per  cent.  Such  a  concentration  by  no  means 
suffices  to  sterilize  the  stomach-contents,  but  in  all  prob- 
ability it  destroys  many  kinds  of  bacteria,  including  some 
which  might  become  the  cause  of  disease.     Others  it  un- 


GASTRIC.   SECRETION   AND    DIGESTION  81 

doubtedly  weakens,  so  that  they  multiply  less  rapidly 
when  the  chyme  passes  on  to  the  small  intestine,  where  the 
conditions  for  bacterial  growth  are  more  favorable.  It 
is  not  surprising  to  find  that  the  species  of  organisms  which 
thrive  most  in  the  stomach  are  those  which  themselves 
produce  acid.  The  '^  sour  stomach  "  commonly  referred  to 
is  a  stomach  in  which  the  generation  of  lactic  acid  from 
sugars  is  actively  taking  place.  This  is  a  process  very  like 
the  familiar  souring  of  milk.  Indeed,  milk  is  one  source  of 
such  acid  fermentation  in  the  stomach.  Excessive  acidity, 
whether  due  to  the  native  juice  or  to  the  activity  of  bac- 
teria, may  be  a  cause  of  discomfort  and  a  hindrance  to 
digestion.  Cannon  has  lately  shown  that  acidity  above 
a  certain  degree  delays  the  departure  of  food  from  the 
stomach,  and  this  is  easily  comprehensible  when  it  is  re- 
called that  after  each  brief  relaxation  the  pylorus  remains 
closed  until  the  acid  which  has  just  passed  has  been 
neutralized  or  dispersed.  Acidity  within  limits  is  a  neces- 
sary condition  of  gastric  digestion,  and  this  will  be  dis- 
cussed later. 

The  Secretion  of  the  Gastric  Juice. — The  glands  of  the 
empty  stomach  seem  to  be  quite  inactive.  The  natural 
supposition  that  they  begin  to  secrete  when  food  comes  in 
contact  with  the  mucous  membrane  is  not  borne  out  by  the 
results  of  experiments.  It  is  most  important  to  note  that 
the  juice  may  start  somewhat  in  advance  of  the  arrival  of 
food.  The  stomach  as  well  as  the  mouth  may  be  said  to 
water  at  the  contemplation  of  a  meal.  Abundant  evi- 
dence of  this  fact  has  been  furnished  by  the  extraordinary 
experiments  of  the  Russian  physiologist  Pawlow  upon 
dogs.  When  a  permanent  opening  has  been  made  to  the 
interior  of  a  dog's  stomach  a  little  gastric  juice  may  issue 
when  the  dog  is  merely  shown  food  which  he  likes. 

That  it  is  unnecessary  to  have  actual  contact  with  the 
stomach  wall  is  still  better  shown  in  the  case  now  to  be 
described.  A  dog  having  a  gastric  fistula  is  subjected  to  a 
second  operation,  by  which  the  esophagus  is  severed  and 
the  portion  connected  with  the  pharynx  made  to  open 

6 


82  NUTRITIONAL   PHYSIOLOGY 

through  the  skin  of  the  neck.  Whatever  is  swallowed  by 
the  dog  is  now  returned  to  the  exterior.  The  pleasure  of 
eating  is  not  impaired.  To  maintain  nutrition  suitable 
food  may  be  introduced  directly  into  the  stomach.  When 
the  dog  chews  and  swallows  a  meal  he  is  quaintly  said  to 
have  a  "Scheinfiitterung" — a  fictitious  feeding.  This 
proceeding  is  accompanied  by  a  steady  flow  of  the  secre- 
tion from  the  gastric  fistula. 

The  secretion  obtained  under  circumstances  like  the 
above  is  called  the  psychic  secretion.  This  term  serves  to 
emphasize  the  fact  that  a  mental  element  is  a  necessary 
incident  of  the  reaction.  The  conditions  governing  gastric 
secretion  seem  to  be  quite  parallel  with  those  which  regu- 
late the  movements  of  the  stomach,  and  their  importance 
in  hygiene  is  equally  evident.  There  are  a  number  of 
individuals  who  have  in  various  ways  lost  the  power  to 
swallow  food,  commonly  because  of  the  closure  of  the 
esophagus.  Their  lives  are  preserved  by  feeding  through 
gastric  fistulas  established  by  surgery.  These  unfortu- 
nates find  it  to  their  advantage  to  attend  to  the  idea  of 
eating,  and  to  taste  and  chew^  portions  of  their  food  at  the 
time  when  it  is  being  introduced  into  their  stomachs. 

After  a  brief  period  of  fictitious  feeding  the  flow  of 
gastric  juice  into  the  stomach  of  a  dog  may  continue  for 
two  or  three  hours.  In  view  of  this  we  must  conclude 
that,  however  important  the  mental  state  may  be  for  the 
initiation  of  the  process,  it  need  not  be  its  accompaniment 
throughout.  We  cannot  pretend  that  a  meal  receives  our 
constant  attention  during  any  such  interval.  Neverthe- 
less we  are  hardly  likely  to  overestimate  the  necessity  of 
securing  a  normal  start.  If  the  initial  circumstances  are 
not  favorable  the  secretion  may  be  long  delayed  or  even 
lacking. 

Now  that  we  have  reserved  a  due  place  for  the  psychic 
element,  we  must  pass  on  to  consider  the  other  factors 
which  modify  the  activity  of  the  glands  of  the  stomach. 
Much  has  been  learned  from  the  surprising  operations  of 
Pawlow  and  others,  who  have  succeeded  in  dividing  the 


GASTRIC    SECRETION    AND    DIGESTION  83 

stomach  of  the  dog  into  two  parts  without  robbing  either 
of  its  connection  with  the  circulatory  and  nervous  systems. 
When  the  dog  has  recovered  from  the  immediate  effects 
it  may  be  said  to  have  two  stomachs.  Either  or  both  may 
communicate  with  the  exterior  by  a  fistula,  while  one  still 
retains  its  normal  relations  with  the  esophagus  and  small 
intestine.  This  arrangement  makes  it  possible  to  place 
food  in  one  stomach  and  to  obtain  and  measure  the  un- 
mixed juice  secreted  into  the  other.  The  evidence  goes 
to  show  that  when  the  glands  in  the  lining  of  one  sac  are 
active  there  is  corresponding  activity  on  the  part  of  those 
in  the  other. 

A  most  significant  fact  is  at  once  noted  when  such  a  dog 
is  under  observation.  There  are  kinds  of  food,  perfectly 
appropriate  for  the  animal's  nutrition,  which  may  lie  in 
the  stomach  without  exciting  any  flow  of  the  juice.  This 
is  said  to  be  true  of  bread,  white  of  egg,  starch,  and  some 
sugars.  The  same  articles  of  diet  would  be  met  by  an 
abundant  gastric  secretion  if  they  had  been  eaten  with 
enjoyment  by  the  hungry  dog.  It  makes  a  radical  difTer- 
ence,  then,  whether  these  materials  enter  the  stomach 
through  the  mouth  and  attract  the  favorable  notice  of 
the  animal,  or  whether  they  are  slipped  through  a  fistula, 
a  proceeding  which  would  probably  not  be  recognized  by 
him  as  a  mode  of  feeding. 

On  the  other  hand,  there  are  some  things  which  do  cause 
an  outbreak  of  gastric  juice  by  their  mere  presence  in  the 
stomach  and  in  the  absence  of  any  psychic  factor.  To  a 
somewhat  limited  extent  this  is  the  case  with  water, 
though  only,  it  appears,  when  there  is  enough  of  it  to  dis- 
tend the  stomach  slightly.  The  best-known  excitant  of 
the  secretion  is  meat,  and  the  property  is  said  to  belong  to 
the  extractives  or  minor  substances  in  this  food,  and  not  to 
the  proteins  of  which  it  is  chiefly  composed.  Meat  causes 
a  considerable  flow  of  the  juice,  but  there  is  a  much  longer 
initial  delay  than  when  the  psychic  element  has  its  normal 
place.  In  fact,  the  total  quantity  produced  when  meat  is 
introduced  into  a  dog's  stomach  without  attracting  his 


84  NUTRITIONAL   PHYSIOLOGY 

attention  is  decidedly  less  than  when  it  is  eaten  in  the 
natural  way,  and  when  the  psychic  and  chemical  agencies 
are  combined. 

It  is  unfortunate  that  our  knowledge  of  this  matter  has 
been  drawn  so  largely  from  a  carnivorous  animal.  Meat 
might  be  expected  to  stimulate  the  stomach  of  the  dog 
more  surely  than  other  foods.  How  far  the  secretion  may 
be  elicited  by  placing  other  compounds  in  the  stomach  is 
imperfectly  known.  Milk  is  credited  with  some  power  to 
call  it  forth,  but  its  superiority  to  water  seems  doubtful. 
Alcohol  is  said  to  have  a  positive  action  of  the  same  kind. 

It  is  claimed  that  the  dextrins,  the  intermediate  bodies 
produced  in  salivary  digestion  of  starch,  have  the  property 
of  starting  the  gastric  flow.  If  this  is  true  it  is  interesting 
as  establishing  a  connecting  link  between  the  two  successive 
processes,  and  making  it  apparent  how  one  may  tend  to 
insure  the  setting  in  of  the  other  in  due  time.  A  link  of 
this  sort  exists  between  gastric  digestion  and  pancreatic 
secretion,  as  we  shall  have  occasion  to  point  out.  The 
condiments,  such  as  pepper  and  spices,  have  a  reputation 
for  stimulating  the  discharge  of  gastric  juice,  and  un- 
doubtedly do  so  when  they  favorably  affect  the  flavor  of 
the  food.  They  are  known  to  increase  the  blood  flow  in  the 
lining  of  the  stomach,  which  would  perhaps  help  to  con- 
tinue the  secretion  process  when  once  under  way,  but 
whether  they  can  actually  initiate  it  apart  from  their 
psychic  effect  remains  uncertain. 

When  gastric  secretion  is  well  started  there  is  provision 
for  its  maintenance  as  long  as  the  stomach  contains  food. 
It  appears  that  some  of  the  early  products  of  digestion 
act  after  the  manner  of  the  extractive  substances  of  meat 
and  excite  the  glands  to  continued  activity.  The  acid 
itself  has  been  found  to  be  absorbed  by  the  cells  lining  the 
antrum  and  to  set  in  motion  a  train  of  events  leading  to 
the  same  result.  This  form  of  stimulation  will  evidently 
cease  only  with  the  departure  of  the  last  portions  of  the 
acid  chyme.  The  flow  of  gastric  juice  is  retarded  by  fats 
and  by  alkaline  mixtures. 


GASTRIC    SECRETION    AND    DIGESTION  85 

Digestion  in  the  Stomach. — The  gastric  juice  is  usually 
said  to  contain  two  enzymes.  Recent  work  indicates  the 
presence  of  a  third.  The  two  familiar  ones  are  pepsin  and 
rennin.  The  third  is  the  gastric  lipase.  Certain  writers 
have  questioned  whether  we  ought  to  speak  of  pepsin  and 
rennin  as  distinct  individuals,  suggesting  rather  that  there 
is  in  the  secretion  a  single  body  having  two  sets  of  proper- 
ties. We  need  not  enter  into  such  a  discussion;  we  shall 
for  the  present  continue  the  convenient  usage  of  speaking 
of  pepsin  and  rennin  as  two  substances. 

Rennin. — The  fact  that  extracts  of  the  stomach  wall 
cause  the  curdling  or  coagulation  of  milk  has  been  known 
from  very  early  times.  Such  extracts,  usually  derived 
from  the  stomach  of  the  calf,  have  long  been  in  use  in  the 
manufacture  of  cheese.  Rennet  is  the  industrial  term  for 
an  extract  with  this  property;  rennin  is  the  scientific  term 
for  the  supposed  enzyme  contained  in  it.  Cheese  curd 
consists  of  the  bulk  of  the  protein  of  milk  which  has 
undergone  an  obscure  chemical  change  and  has  passed  into 
an  insoluble  form.  From  a  physical  standpoint  this  is  an 
anomaly  among  the  digestive  processes.  We  look  to  see 
solids  becoming  liquids,  while  in  this  curious  instance  a 
liquid  becomes  a  solid.  No  very  convincing  explanation 
of  this  occurrence  has  been  offered.  It  may  be  suggested 
that  it  prevents  an  unduly  rapid  passage  to  the  small  in- 
testine, but  so  far  as  we  know  the  mechanism  of  the 
pylorus  is  entirely  competent,  even  for  liquid  food.  The 
curd  when  formed  has  to  undergo  solution  like  any  other 
soKd. 

The  action  of  rennin  becomes  the  more  enigmatic  when 
it  is  noted  that  it  is  found  in  the  stomachs  of  animals 
which  do  not  have  milk  in  their  normal  diets.  Milk  is 
curdled  by  extracts  of  various  organs  other  than  the  digest- 
ive glands  and  by  some  vegetable  juices.  In  the  human 
stomach  a  very  firm  curd  may  be  formed  when  a  large 
quantity  of  cows'  milk  is  taken  at  one  time.  The  dense 
mass  may  be  slow  to  digest.  Human  milk  is  said  never  to 
set  into  such  a  tenacious  coagulum,  and  this  is  natural, 


86  NUTRITIONAL   PHYSIOLOGY 

since,  regarded  as  a  solution  of  proteins,  it  is  much  more 
dilute  than  the  milk  of  the  cow. 

Peptic  Digestion. — The  chief  enz3niie  of  the  gastric  juice 
is  the  one  commonly  called  pepsin.  Its  relations  with  the 
acid  of  the  stomach  are  so  close  that  many  writers  urge 
that  we  should  speak  rather  of  ' 'pepsin-hydrochloric  acid,'' 
the  term  suggesting  the  existence  of  a  compound  of  the 
two  which  is  responsible  for  the  action  on  the  food.  The 
power  to  digest  proteins  is  manifested  only  with  an  acid 
reaction,  and  is  permanently  lost  when  the  mixture  is  made 
distinctly  alkaline.  The  conditions  which  permit  peptic 
digestion  to  take  place  are,  therefore,  precisely  those  which 
exclude  the  action  of  the  saliva. 

When  protein  in  solid  form,  such  as  boiled  white  of 
egg,  is  subjected  to  the  influence  of  gastric  juice  the  pieces 
swell  and  become  softened.  Later  they  are  dissolved. 
When  the  trial  is  made  with  protein  which  is  originally  in 
solution,  such  as  unboiled  white  of  egg,  there  is  no  visible 
evidence  of  change.  But  there  are  physical  and  chemical 
tests  which  can  be  employed  to  show  that  digestion  is  as 
definite  a  change  in  this  case  as  in  the  other.  An  early 
manifestation  of  this  fact  is  the  loss  of  the  property  of 
coagulation  on  heating.  Later  there  are  indications  that 
the  molecules  are  undergoing  cleavage.  At  each  successive 
stage  there  is  a  gain  in  the  power  of  diffusion,  a  reduction 
of  viscosity,  and  a  diminution  in  the  number  of  precipitants 
which  can  be  employed  to  throw  the  protein  out  of  solution. 

Physiologic  chemists  have  studied  minutely  the  charac- 
teristics of  the  hydrolytic  products  during  the  advance  of 
peptic  digestion.  They  have  attempted  to  identify  nu- 
merous compounds,  each  of  which  has  a  transient  existence 
and  is  then  itself  hydrolyzed.  For  our  present'  purposes  it 
would  be  unprofitable  to  dwell  upon  such  questions  of  detail. 
Certain  of  the  earlier  cleavage  products  are  included  under 
the  general  name  of  proteoses  or  alhiimoses;  others,  arising 
later  and  of  a  simpler  character,  are  called  peptones. 
Roughly  speaking,  there  is  a  parallel  between  salivary  and 
peptic  digestion.     In  either  case,  molecules  of  great  size — 


GASTRIC    SECRETION    AND    DIGESTION  87 

of  starch  and  protein  respectively — are  subdivided  pro- 
gressively, first  with  the  formation  of  somewhat  complex 
bodies — dextrins  and  proteoses — later  to  form  maltose  in 
the  first  instance  and  peptones  in  the  second.  The  corre- 
spondence is  imperfect  in  several  ways;  for  example,  mal- 
tose is  a  single  substance,  while  there  appear  to  be  a  number 
of  peptones.  It  will  be  remembered  that  maltose  itself 
is  slowly  transformed  by  long-continued  action  of  saliva. 
Quite  similarly,  the  gastric  juice  will  in  time  effect  a  fur- 
ther digestion  of  the  peptones,  but  this  advanced  diges- 
iioD.  seems  normally  to  be  postponed  until  the  intestine  is 
reached. 

Gastric  Lipase. — Down  to  a  recent  time  it  was  held  that 
fat  underwent  no  true  digestion  in  the  stomach.  It  was 
recognized  that  some  forms  of  fat,  butter,  for  example, 
must  be  melted,  and  that  the  fat  in  the  adipose  tissue  of 
meat  might  be  released  from  the  enclosing  cells  w^hen  their 
protein  portion  was  dissolved.  These,  however,  are  mere 
physical  changes.  Attentive  study  has  brought  out  the 
fact  that  there  is,  after  all,  some  hydrolysis  of  fat  in  the 
stomach,  though  it  is  probably  slight.  An  enzyme  with 
this  action  is  accordingly  assumed  and  is  spoken  of  as 
gastric  lipase.  It  is  only  the  most  finely  divided  (emulsi- 
fied) fats  which  seem  to  be  appreciably  affected.  The 
products  of  this  decomposition  as  well  as  of  the  later  fat 
digestion  in  the  intestine  are  glycerin  and  free  fatty  acids. 

Summary. — The  material  passing  the  pylorus  is  com- 
paratively dilute  and  normally  free  from  coarse  particles. 
It  is  acid  in  reaction,  both  the  native  hydrochloric  acid  and 
the  acids  formed  by  fermentation  contributing.  Much 
of  the  food  is  as  yet  practically  undigested.  On  the  other 
hand,  some  progress  has  been  made  in  the  transformation 
of  cooked  starch  into  sugar.  The  proteins  are  partially 
peptonized.  If  milk  has  formed  a  part  of  the  diet,  it  will 
have  been  curdled  and  redissolved.  Fats  may  have  been 
liquefied  and  scattered,  but  are  not  likely  to  have  been 
extensively  hydrolyzed.  On  the  whole,  gastric  digestion 
may  fairly  be  described  as  preliminary  in  character. 


CHAPTER  X 

THE     SMALL     INTESTINE:     ITS     MOVEMENTS, 
SECRETIONS,  AND   DIGESTIVE  PROCESSES 

The  intestinal  content  is  propelled  on  its  winding  way 
by  peristaltic  movements.  These  are  similar  in  their 
mechanical  principle  to  the  waves  of  muscular  contraction 
passing  down  the  esophagus  in  the  act  of  swallowing. 
They  are,  however,  much  slower  and  gentler  in  character. 
As  a  rule  they  do  not  run  an  uninterrupted  course  from  the 
pylorus  to  the  ileocecal  valve,  though  such  a  phenomenon 
is  an  occasional  possibility.  More  commonly  a  wave  will 
travel  a  limited  distance  and  then  fade  out,  leaving  the 
material  which  it  was  moving  to  rest  for  a  time  in  some 
depending  loop.  If  this  is  true,  the  progress  of  the  food  is 
intermittent;  x-ray  observations  upon  human  subjects 
have  led  to  the  estimate  that  it  takes  four  or  five  hours  for 
the  passage  through  the  whole  length  of  the  small  intestine. 
This  time  may  be  assumed  to  vary  widely  with  the  indi- 
vidual and  the  diet.  Reduced  to  an  average  rate  the  data 
quoted  above  give  us  about  one  inch  per  minute. 

Intestinal  peristalsis,  like  the  movements  of  the  stom- 
ach, is  governed  mainly  by  local  mechanisms.  Yet  in  this 
case  as  in  the  other  the  central  nervous  system  may  exert 
an  influence  tending  to  accelerate  or  to  suppress  the  ac- 
tivity of  the  muscular  coats.  The  second  or  inhibitory 
action  is  the  more  marked.  A  question  much  discussed 
is  whether  the  direction  of  peristalsis  in  the  small  intestine 
is  at  all  subject  to  reversal.  The  sum  of  the  evidence  at 
present  supports  the  view  that  such  a  reversal  (antiperis- 
talsis)  is  possible,  but  only  under  conditions  which  are 
clearly  abnormal.  Ordinarily  it  is  plain  that  the  intestine 
is  distinctly  specialized  to  act  in  one  way  rather  than  the 
other. 

88 


THE    SMALL    LXTESTIXE  89 

Not  all  the  muscular  contractions  exhibited  by  the  small 
intestine  have  a  progressive  character.  Frequently  a  loop 
which  contains  food  will  become  creased  at  short  intervals 
by  rings  of  constriction  which  do  not  shift  their  position, 
but  remain  stationary  for  a  time.  The  internal  effect  is  to 
create  a  series  of  small  pouches  holding  separate  portions 
of  the  chyme.  After  this  condition  has  persisted  for  a 
time  the  regions  originally  contracted  become  relaxed,  and 
new  contractions  set  in  at  points  midway  between  them. 
Under  the  influence  of  such  movements  the  food  is  con- 
stantly shifted  about  and  subdivided,  but  it  is  not  driven 
steadily  in  one  direction.  This  ''marking  time"  on  the 
part  of  the  small  intestine  is  referred  to  as  rhythmic  seg- 
mentation. Inasmuch  as  it  serves  to  alter  the  contact 
relations  between  the  intestinal  contents  and  the  lining, 
it  probably  favors  absorption.  Some  writers  have  made 
much  of  the  effect  which  these  contractions  may  be  as- 
sumed to  have  upon  the  flow  of  blood  and  lymph  in  the 
walls  of  the  canal.  When  pressure  is  applied  at  brief 
intervals  to  tissue  containing  these  fluids  the  result  may  be 
described  as  a  massaging  action,  hastening  the  circulation 
and  crowding  out  some  of  the  lymph.  This  again  must 
tend  to  promote  the  absorption  process. 

A  longitudinal  movement  of  individual  loops  is  often 
described.  Two  neighboring  turns  of  the  intestine  may  be 
seen  to  glide  the  one  upon  the  other,  coming  to  rest  after 
slipping  a  short  distance,  and  presently  reversing  the  direc- 
tion of  their  travel.  This  form  of  activity  cannot  defi- 
nitely further  the  progress  of  the  contents,  but  when  it 
affects  segments  enclosing  liquid  material  we  may  suppose 
that  the  food  and  juices  are  tilted  about  and  brought  into 
relation  Avith  the  largest  possible  area  of  absorbing  surface. 

The  Secretions  Entering  the  Small  Intestine. — In 
Chapter  VI  it  was  stated  that  this  part  of  the  canal  re- 
ceives the  contributions  of  the  Hver  and  the  pancreas,  as 
well  as  of  the  microscopic  glands  in  its  own  mucous  mem- 
brane. The  bile  and  the  pancreatic  juice,  it  will  be  re- 
membered, enter  just  below  the  pylorus.     The  intestinal 


90  NUTRITIONAL    PHYSIOLOGY 

juice  is  produced  by  all  parts  of  the  extensive  lining,  but 
more  abundantly  in  the  upper  than  in  the  lower  segments. 
The  three  secretions  have  some  characters  in  common. 
They  are  all  alkaline  in  reaction,  owing  to  the  presence  in 
them  of  sodium  carbonate.  This  confers  upon  them  in  a 
considerable  degree  the  power  to  neutralize  acids.  As 
the  acid  chyme  from  the  stomach  meets  the  alkaline  secre- 
tions in  the  duodenum  there  must  be  more  or  less  carbonic 
acid  gas  evolved.  This  may  be  helpful  to  the  digestive 
process,  since  the  tendency  will  be  to  lighten  the  texture 
of  the  food  particles,  much  as  dough  is  lightened  by  the 
agency  of  yeast. 

It  is  not  merely  the  acid  from  the  stomach  which  may  be 
combated  by  the  alkali  of  the  juices  below;  there  are  two 
other  sources  of  acid  to  be  taken  into  account.  One  of 
these  is  found  in  the  bacterial  fermentation,  chiefly  of 
sugars,  which  goes  on  in  the  intestine.  The  second  is  the 
entirely  normal  formation  of  free  acids  occurring  in  the 
course  of  fat  digestion.  So  far  as  the  first  class  of  acids  are 
neutralized  the  products  are  mainly  lactates  and  buty- 
rates;  the  fatty  acids  may  be  converted  into  soaps.  There 
is  no  guarantee  of  an  exact  proportionality  between  the 
acids  and  the  alkali,  and  it  is  impossible  to  say  which  will 
be  in  excess  in  a  particular  part  of  the  canal.  Generally, 
however,  the  resulting  reaction  of  the  mixture  is  not  far 
from  the  neutral  point. 

The  united  volume  of  the  three  secretions  is  held  to  be 
very  large,  but  any  estimate  tends  to  mislead,  since 
throughout  the  length  of  the  intestine  we  have  the  with- 
drawal of  water  keeping  pace  approximately  with  its  in- 
flow. In  a  certain  section  selected  for  observation  the 
bulk  of  the  contents  may  show  little  change  during  a  long 
period,  and  yet  there  may  have  been  profuse  secretion 
entirely  disguised  by  counterbalancing  absorption.  In 
this  consideration  we  see  indicated  an  important  service 
shared  by  all  the  digestive  secretions,  that  of  supplying 
liberal  quantities  of  water  to  act  as  the  solvent  and  carrier 
of  food-stuffs  destined  for  absorption.     If  absorption  were 


THE    SMALL    INTESTINE  91 

to  continue  without  compensatory  secretion,  a  final  stage 
might  be  reached  in  whicii  the  cavity  of  the  intestine  would 
be  drained  of  all  fluid,  while  the  walls  would  be  crusted 
with  a  dry  residue  suggestive  of  boiler  scale. 

The  Pancreatic  Juice. — In  considering  the  causes  of 
gastric  secretion  the  importance  of  the  central  nervous 
system  called  for  emphasis.  This  is  not  true  to  the  same 
extent  of  the  government  of  the  pancreas,  though  here 
also  it  is  held  that  the  nervous  system  plays  a  part.  A 
chemical  means  of  control  is  better  kno^\^l.  As  has  been 
hinted  before,  there  is  an  intimate  relation  between  the 
gastric  activity  and  the  later  awakening  of  the  pancreas 
from  its  usual  state  of  repose.  The  arrival  of  acid  from 
the  stomach  in  the  duodenum  causes  a  timely  outflow  of 
pancreatic  juice. 

This  might  be  supposed  to  be  an  instance  of  reflex  action, 
like  the  production  of  saliva  when  acid  is  taken  into  the 
mouth.  It  has  been  shown,  however,  that  it  has  another 
explanation.  The  acid,  striking  into  the  lining  membrane 
of  the  duodenum,  initiates  a  series  of  reactions  which  have 
been  studied  in  detail,  and  which  lead  at  last  to  the  for- 
mation of  a  substance  of  quite  definite  chemical  properties 
called  secretin.  This  finds  its  way  into  the  circulation, 
and,  like  a  drug,  is  swept  far  and  wide.  It  stimulates  the 
pancreas  to  produce  its  secretion,  augments  the  formation 
of  bile  by  the  liver,  and  probably  excites  the  glands  in  the 
wall  of  the  intestine  to  greater  activity.  In  the  light  of 
these  facts  it  becomes  clear  that  a  vigorous  gastric  digestion 
with  the  strongly  acid  chyme  which  results  goes  far  to 
insure  a  normal  intestinal  process. 

The  pancreatic  juice  in  man  is  an  abundant  secretion — 
a  pint  or  more  daily — and,  in  marked  contrast  with  the 
saliva  and  the  gastric  juice,  it  has  relations  with  all  three 
principal  classes  of  food-stuffs.  It  contains  an  enzyme 
which  some  have  thought  to  be  identical  with  the  ptyalin 
of  the  saliva,  but  generally  called  amylopsin  or  pancreatic 
amylase.  Thus  the  progress  of  starch  digestion,  inter- 
rupted for  a  time  in  the  stomach,  is  now  renewed.     The 


92  NUTRITIONAL   PHYSIOLOGY 

slight  acidity  which  may  exist  in  the  intestine  is  not  hkely 
to  prevent  this  type  of  digestion  from  going  on  to  com- 
pletion. 

Beside  acting  on  starches,  the  pancreatic  juice  continues 
and  greatly  accelerates  the  hydrolysis  of  fats  which  has 
been  barely  begim  in  the  stomach.  The  enzyme  concerned 
is  called  steapsin  in  the  older  books,  but  by  more  recent 
writers  lipase.  The  immediate  products  are  glycerin  and 
fatty  acids.  A  secondary  formation  of  soaps  is  a  possibil- 
ity already  indicated.  When  an  oil  undergoes  digestion 
it  breaks  up  at  an  early  stage  into  microscopic  drops  and  is 
said  to  be  emulsified.  This  subdivision  clearly  multiplies 
the  surface  of  contact  between  the  food  and  the  digestive 
juice,  and  has  an  effect  corresponding  to  that  of  mastication 
upon  solids.  It  is,  therefore,  most  helpful  to  digestion, 
but  it  is  not  to  be  confused  with  digestion  itself. 

The  statement  is  commonly  made  that  the  pancreatic 
juice  continues  the  work  of  pepsin  upon  proteins  by  virtue 
of  an  enzyme  called  trypsin.  While  this  is  approximately 
true,  it  calls  for  a  certain  qualification.  If  the  juice  is 
carefully  collected  as  it  comes  from  the  main  duct  of  the 
pancreas  without  being  allowed  to  mingle  with  other  se- 
cretions or  even  to  touch  the  lining  of  the  intestine,  it  is 
reported  that  it  is  usually  incapable  of  hydrolyzing  pro- 
teins. This  power  it  gains  in  a  striking  degree  when  it  has 
been  mixed  with  ever  so  little  of  the  intestinal  juice,  the 
succus  entericus,  as  it  is  sometimes  called.  The  interaction 
of  the  two  secretions  is  described  as  resulting  in  the 
"activating"  of  the  pancreatic  juice.  The  natural  infer- 
ence is  that  the  inactive  fluid  contains  in  solution  a  body, 
not  yet  deserving  the  name  of  enzyme,  but  ready  to  be- 
come one  by  a  quick  transformation.  An  inactive  ante- 
cedent body  of  this  kind  is  termed  a  zymogen;  in  this  specific 
instance,  trypsinogen.  Acting  on  the  assumption  that 
there  is  a  definite  substance  in  the  intestinal  juice  capable 
of  changing  trypsinogen  to  trypsin,  physiologists  have 
given  the  name  of  enter okinase  to  the  agent  concerned. 

Tryptic  digestion  differs  in  characteristic  ways  from  the 


THE    SMALL    INTESTINE  93 

peptic  process  which  has  been  described.  Acid  which  is 
essential  to  digestion  in  the  stomach  is  antagonistic  to  the 
pancreatic  type.  Trjrpsin  does  its  work  best  in  a  nearly 
neutral  mixture.  In  the  normal  course  of  events  it  acts 
upon  material  already  partly  hydrolyzed,  but  it  has  the 
power  to  carry  through  all  its  stages  the  digestion  of  native 
protein.  If  the  food  is  a  solid,  mere  inspection  reveals  a 
difference  between  peptic  and  tryptic  solution.  In  the 
former  case  there  is  marked  swelling,  as  previously  stated ; 
in  the  latter  there  is  progressive  corrosion,  a  shredding  or 
honeycombing  of  the  specimen. 

When  chemical  methods  are  employed  in  the  study  of 
tryptic  digestion,  it  is  found  to  run  a  course  roughly  parallel 
with  that  of  the  gastric  digestion  of  proteins.  Correspond- 
ing intermediate  bodies — proteoses — are  described  in  both 
instances,  though  it  is  said  that  some  stages  in  the  tryptic 
process  are  passed  so  rapidly  as  to  seem  almost  to  be  omit- 
ted. What  distinguishes  the  pancreatic  proteolysis  most 
radically  from  the  peptic  is  the  facility  with  which  the 
peptones  are  broken  down  into  still  simpler  bodies.  We 
have  made  the  statement  that  these  late  cleavages  take 
place  only  very  slowly  under  the  influence  of  gastric  juice. 
So  it  becomes  clear  that  trypsin  is  distinctly  adapted  to 
follow  after  pepsin  in  the  accomplishment  of  protein  di- 
gestion. 

Peptones  are  compounds  which  are  simple  by  comparison 
with  standard  proteins,  but  which  are  still  too  complex 
to  be  given  precise  chemical  formulas.  When  they  are 
hydrolyzed,  most  of  the  products  come  within  the  knowl- 
edge of  the  organic  chemist  so  definitely  that  their  molecu- 
lar structure  can  be  confidently  expressed.  To  the  student 
it  must  be  admitted  that  such  formulas  do  not  appear 
simple,  but  if  he  is  disposed  to  resent  the  use  of  the  word 
he  is  to  reflect  that  these  molecules  stand  in  some  such  rela- 
tion to  the  original  protein  complex  as  the  bits  of  the  mosaic 
bear  to  the  whole  design.  It  is  with  some  such  an  idea 
that  the  Germans  have  called  them  Bausteine,  that  is, 
the  building-stones,  from  which  a  new  architecture  can  be 


94  NUTRITIONAL   PHYSIOLOGY 

constructed.  The  simplest  products  of  the  tryptic  process 
are  conveniently  called  amino-acids. 

The  Intestinal  Juice. — This  copious  secretion  was  for- 
merly regarded  as  having  little  to  do  with  digestion.  The 
present  disposition  is  to  credit  it  with  a  very  considerable 
share.  When  the  pancreatic  juice  is  prevented  from  enter- 
ing the  intestine  it  remains  possible  to  keep  up  the  nutri- 
tion of  the  animal,  and  one  must  conclude  that  the  intes- 
tinal juice  is  successfully  preparing  more  than  one  kind 
of  food  for  absorption.  Samples  of  the  secretion  have  often 
been  obtained  from  loops  of  the  intestine  disconnected  from 
the  remainder  of  the  canal.  Different  workers  give  vary- 
ing accounts  of  its  properties. 

The  feature  of  its  digestive  action  concerning  which  there 
is  the  most  general  agreement  is  the  hydrolysis  of  the  more 
complex  sugars,  the  disaccharids.  Of  these  sugars,  three 
are  commonly  present,  and  there  appear  to  be  three  en- 
zymes adapted  to  act  upon  them.  Maltose  arises  princi- 
pally from  the  salivary  and  pancreatic  digestion  of  starch. 
It  is  hydrolyzed  to  dextrose  (glucose)  by  an  enzyme,  which 
is  best  called  maltase.  Lactose,  or  milk-sugar,  is  similarly 
converted  into  equal  parts  of  dextrose,  and  the  less  familiar 
sugar  galactose  by  the  enzyme  lactase.  Saccharose,  or 
cane-sugar,  gives  rise  to  dextrose,  and  a  sugar  of  different 
properties,  levulose  (fructose),  under  the  influence  of  the 
enzyme,  invertase. 

When  an  extract  is  prepared  from  the  thoroughly  minced 
lining  of  the  small  intestine  it  can  be  shown  to  have  the 
power  to  cause  proteoses  and  peptones  to  undergo  hydro- 
lysis, though  it  is  said  not  to  act  upon  the  original  unmodi- 
fied protein.  This  is  equivalent  to  saying  that  such  an 
extract  can  parallel  the  later  work  of  trypsin,  though  lack- 
ing its  power  to  initiate  digestion.  The  enzyme  implied 
has  been  named  erepsin.  It  is  regarded  as  an  open  ques- 
tion whether  this  enzyme  normally  enters  the  cavity  of  the 
intestine  or  does  its  work  within  the  confines  of  the  cells 
from  which  it  can  be  extracted.  We  may  conceive  that 
when  an  animal  is  deprived  of  its  pancreatic  secretion  it  is 


THE    SMALL   INTESTINE  95 

still  able  to  digest  proteins,  pepsin  beginning  the  digestion 
and  carrying  it  to  a  stage  at  which  the  products  are  suscep- 
tible to  the  action  of  erepsin.  The  presence  of  enteroki- 
nase  in  the  intestinal  juice  has  just  been  noted. 

The  Bile. — The  secretion  of  the  liver  cannot  be  regarded 
in  the  same  light  as  the  digestive  juices  mentioned  hereto- 
fore. It  is  not  secreted  merely  after  meals,  but  is  always 
flowing  through  the  ducts  which  converge  from  the  several 
lobes  of  the  liver.  It  is  not  necessarily  entering  the  in- 
testine at  all  times,  since  the  gall-bladder  provides  a  place 
for  its  temporary  storage,  as  described  in  Chapter  VI. 
While  the  production  of  bile  never  ceases,  it  does  show  an 
acceleration  during  the  digestive  periods,  and  this  is  be- 
lieved to  be  in  response  to  the  stimulating  effect  of  secretin. 

Bile  attracted  the  attention  of  physicians  in  very  early 
times,  its  bright  color  and  intensely  bitter  taste  giving  it 
a  certain  distinction.  It  entered  largely  into  ancient 
theories  of  disease  and  of  medicine.  We  have  traces  of 
these  facts  in  the  root-meaning  of  such  words  as  bilious, 
choleric,  and  melancholy.  In  popular  estimation  bile  is 
a  poison  arising  now  and  then  in  the  system  and  causing 
digestive  disturbances.  Patient  study  has  shown  that  the 
bile  is  a  complex  mixture,  and  that  it  numbers  among  its 
constituents  some  which  are  waste-products  and  others 
which  have  a  favorable  effect  upon  the  progress  of  diges- 
tion and  absorption.  It  stands,  therefore,  in  a  position 
intermediate  between  that  of  the  gastric  juice,  which  is 
formed  solely  to  advance  digestion,  and  that  of  the  urine, 
which  is  composed  of  material  useless  to  the  body. 

The  pigments  of  the  bile  are  counted  as  waste  substances. 
A  red  one  predominates  in  carnivorous  animals.  The  bile 
of  the  herbivora  is  green,  as  is  also  the  case  with  human  bile. 
These  pigments  show  in  their  chemical  nature  a  close  rela- 
tionship with  the  red  coloring-matter  of  the  blood,  the 
important  compound  hemoglobin.  All  the  evidence  goes 
to  show  that  the  bile-pigments  are  modified  fractions  of  the 
great  hemoglobin  molecule,  and  that  their  abundance  is 
an  indication  of  the  amount  of  destruction  suffered  by  the 
red  corpuscles  of  the  blood.     These  pigments  are  relatively 


96  NUTRITIONAL   PHYSIOLOGY 

insoluble  and  are  not  always  successfully  carried  to  the 
intestine  by  the  bile.  When  they  deposit  in  solid  form  in 
the  gall-bladder  they  contribute  to  the  formation  of  ''gall- 
stones/' aggregations  in  which  another  waste-product, 
the  waxy  cholesterin,  may  be  included.  The  pigments  are 
usually  more  or  less  altered  by  bacterial  action  in  the 
course  of  their  journey  through  the  canal,  and  eventually 
become  the  chief  coloring-matter  of  the  feces. 

The  bitter  taste  of  bile  is  due  to  two  organic  salts  of 
high  molecular  weight.  These  seem  to  have  a  totally  dif- 
ferent significance  from  that  of  the  pigments  and  choles- 
terin. They  are  not  lost  to  the  body,  but  are  absorbed 
from  the  lower  part  of  the  small  intestine  and  are  presum- 
ably secreted  again  and  again.  This  phenomenon  has 
been  spoken  of  as  the  "circulation  of  the  bile-salts."  The 
withholding  of  these  bodies  from  the  alimentary  tract  tends 
to  derange  digestion  and,  in  particular,  to  diminish  the 
absorption  of  fats.  When  bile  is  tested  by  itself  it  shows 
only  the  feeblest  digestive  powers,  yet  pancreatic  digestion 
is  greatly  promoted  by  its  presence,  and  it  may  be  likewise 
an  ally  of  the  intestinal  juice. 

Light  is  thrown  on  the  properties  of  the  bile  by  observing 
the  condition  of  jaundice.  This  disorder  is  commonly 
caused  by  the  more  or  less  general  plugging  of  the  bile- 
ducts  with  mucus.  The  secretion  cannot  make  its  escape 
from  the  liver  in  the  normal  way  and  some  of  it  enters  the 
circulation.  Bile-pigments  make  their  appearance  in  the 
white  of  the  eye  and  in  the  skin.  The  urinary  pigment, 
which  is  always  closely  related  to  the  pigments  of  the  bile, 
is  much  increased.  The  ill  feeling  which  usually  attends 
the  condition  may  be  due  in  part  to  the  mildly  poisonous 
effect  of  the  abnormally  retained  bile  constituents.  It  is 
likely  to  be  aggravated  by  indigestion.  The  bile-salts  are 
lacking  and  the  capacity  to  digest  the  food  and  promptly 
absorb  the  end-products  is  greatly  reduced.  Bacterial 
action  in  the  intestine  may  become  pronounced.  This 
last  fact  has  suggested  that  the  bile  may  be  an  antiseptic. 
It  cannot  be  shown  to  have  anything  like  a  universal  action 
of  this  kind,  but  it  is  very  probable  that  it  has  a  selective 


THE    SMALL    INTESTINE  97 

one,  favoring  one  type  of  organism  and  restraining  another. 
Even  though  it  had  no  such  influence,  the  intestinal 
bacteria  might  be  expected  to  multiply  in  its  absence,  for 
the  simple  fact  of  delayed  absorption  would  suffice  to  bring 
this  about.  Our  best  defense  against  excessive  fermenta- 
tion and  putrefaction  is  in  the  early  and  complete  removal 
of  the  food  from  the  sphere  of  action  of  micro-organisms. 

Summary. — The  secretions  flowing  into  the  small  intes- 
tine supply  enzymes  in  sufficient  number  and  variety  to 
accomplish  the  digestion  of  all  common  foods.  The  trans- 
formation of  starch  to  maltose  begun  in  the  stomach  is 
completed  by  the  pancreatic  amylase.  The  resolution  of 
proteins  into  the  simple  structural  units  from  which  their 
molecules  are  built  is  carried  out  under  the  influence  of 
trypsin  and  perhaps  of  erepsin.  The  last-named  enzyme 
is  supposed  by  many  to  work  upon  the  tryptic  products  as 
they  pass  through  the  lining  cells  on  their  way  to  the  blood. 
Fats  are  hydrolyzed  by  the  pancreatic  lipase,  with  the 
formation  of  glycerin  and  fatty  acids,  the  latter  being 
in  some  measure  converted  to  soaps.  The  disaccharids  are 
changed  to  monosaccharids,  a  work  attributed  to  the  in- 
testinal juice.  Fermentation  caused  by  bacteria  has  been 
taking  place  along  with  the  strictly  normal  processes. 
The  most  evident  products  are  organic  acids,  which  may  or 
may  not  be  fully  neutralized  by  the  alkaline  secretions. 

Protection  Against  Self-digestion. — There  has  been 
much  discussion  of  the  fact  that  the  proteolytic  enzymes 
in  the  digestive  tract  do  not  ordinarily  attack  its  mucous 
membrane.  They  may  do  so  after  death  and  when  the 
tissues  are  in  an  abnormal  condition,  as  in  case  of  gastric 
ulcer,  the  juices  may  strongly  antagonize  the  healing  pro- 
cess. It  is  often  asserted  that  there  is  a  definite  chemical 
difference  between  the  proteins  of  living  and  of  lifeless 
matter.  A  recent  explanation  of  the  resistance  which 
living  cells  offer  to  digestion  is  based  on  the  apparent  fact 
that  such  cells  form  bodies  which  have  the  capacity  to  neu- 
tralize enzymes  in  fixed  proportions.  The  name  of  anti- 
enzymes  has  been  applied  to  such  protective  substances. 
7 


CHAPTER  XI 
THE  LARGE  INTESTINE 

The  material  passing  into  the  colon  is  dilute  ^nd  much 
reduced  in  volume  as  compared  with  the  chyme  passing  out 
of  the  stomach.  In  the  lower  part  of  the  small  intestine 
the  absorption  of  water  more  than  counterbalances  the 
secretion,  hence  the  shrinkage  of  the  contents.  But 
since  the  end-products  of  digestion  are  being  absorbed  also, 
there  is  no  tendency  toward  extreme  concentration.  In 
the  large  intestine  there  is  but  little  secretion,  and  the  con- 
tinuation of  the  absorption  of  water  reduces  the  contents 
at  last  to  a  nearly  solid  consistency. 

The  colon  appears  to  be  of  very  unequal  value  to  animals 
of  different  classes.  In  the  carnivora  the  work  of  diges- 
tion and  absorption  is  so  nearly  finished  by  the  small  in- 
testine that  very  little  remains  to  be  done.  The  small 
quantity  of  matter  having  a  potential  food  value  may  be 
accompanied  into  the  large  intestine  by  enzymes,  which 
may  there  carry  further  their  digestive  action.  Such  food, 
however,  is  likely  to  be  a  negligible  amount,  and  the  digest- 
ive powers  of  the  mixture  at  this  point  are  unreliable. 
In  the  herbivora,  the  food  being  bulky  and  refractory, 
a  considerable  portion  may  arrive  undigested  in  the  colon. 
These  animals  have  very  capacious  ceca,  in  which  great 
masses  of  contents  seem  to  be  held  for  long  intervals. 
The  digestion  occurring  there  may  be  partly  effected  by 
the  native  secretions,  but  it  is  believed  to  be  largely  the 
work  of  bacteria. 

The  average  human  being  resembles  the  carnivorous 
type  rather  than  the  other.  Numerous  cases  have  been 
observed  in  which  no  use  was  made  of  the  colon,  and  it  was 
never  difficult  to  maintain  nutrition.     When  the  large  in- 

98 


THE    LARGE    INTESTINE 


99 


testine  is  no  longer  traversed  the  discharges  are  watery  and 
rather  voluminous,  but  they  contain  only  small  percentages 


Fig.  14. — The  colon  with  special  reference  to  its  movements: 
T  is  placed  near  the  seat  of  frequent  sustained  or  tonic  contraction. 
From  here  backward  to  the  cecum  (C)  antiperistalsis  is  of  common 
occurrence,  but  this  segment  is  swept  also  by  occasional  waves  in 
the  opposite  direction ;  V  indicates  the  position  of  the  ileocecal  valve, 
which  prevents  reflux  to  the  small  intestine  under  the  influence  of 
antiperistalsis;  beyond  T  the  infrequent  movements  which  take  place 
have  always  a  progressive  character;  S  and  S^  are  regions  often 
found  to  contain  stationary  contents;  K  is  the  kink  referred  to  in 
the  text;  R  is  the  rectum,  and  S-ph  is  the  double  sphincter  at  the 
anus. 


of  valuable  nutriment.  The  fluid  escaping  from  the  end  of 
the  small  intestine  or  the  beginning  of  the  large  is  described 
as  entirely  inoffensive,  which  indicates  that  putrefactive 


100  NUTRITIONAL   PHYSIOLOGY 

decomposition  of  protein  is  usually  confined  to  the  colon. 
Protein  putrefaction  is  nowadays  held  accountable  for 
many  ailments,  and  from  this  view  has  arisen  the  common 
teaching  that  man  would  be  better  off  without  a  large  in- 
testine. Comment  upon  this  opinion  may  well  be  re- 
served for  the  chapter  on  the  Hygiene  of  Nutrition. 

The  Movements  of  the  Colon. — The  average  rate  of 
progress  in  the  large  intestine  is  low.  A  factor  tending  to 
make  it  so  has  recently  been  brought  to  notice  through  the 
studies  of  Cannon  and  others.  The  ascending  colon  has  a 
property  not  duplicated  elsewhere  in  the  canal,  that  of 
habitual  antiperistalsis.  As  repeated  instalments  pass  the 
ileocecal  valve  the  cecum  is  filled  and  the  accumulating 
contents  reach  higher  and  higher  levels,  but  instead  of 
thrusting  onward  this  gathering  mass,  the  circular  mus- 
cular coat  most  of  the  time  urges  it  backward.  The  result 
is  said  to  be  much  like  what  is  seen  in  the  antrum  of  the 
stomach,  although  here  the  direction  of  the  movement  is 
reversed,  that  is,  the  waves  of  contraction  press  the  mix- 
ture downward  into  a  pouch,  from  which  the  only  escape 
is  by  an  eddying  reflux  through  the  crawling  ring.  It  is 
assumed  that  the  ileocecal  valve  prevents  any  return  to  the 
small  intestine.  A  deliberate  hindrance  to  progress  at  this 
point  may  be  of  service  so  far  as  it  secures  a  more  perfect 
absorption  of  digestive  products  and  the  retrieving  of  sur- 
plus water.  The  antiperistaltic  waves  are  described  as 
starting  from  a  zone  in  the  transverse  colon  near  the  mid- 
line of  the  body. 

From  time  to  time  contractions  sweep  over  the  ascending 
colon  in  what  we  should  call  the  normal  direction,  and  send 
onward  more  or  less  of  its  contents.  Beyond  the  midpoint 
referred  to  above  those  movements  which  occur  are  invari- 
ably progressive,  but  are  separated  by  long  periods  of  re- 
pose. If  we  adopt  a  recent  estimate  we  can  make  the  fol- 
lowing general  statement :  The  foremost  portion  of  a  meal, 
that  which  was  first  to  pass  the  pylorus  and  the  ileocecal 
valve,  may  be  expected  to  be  near  the  spleen  at  the  end  of 
the  ninth  hour.     When  defecation  occurs  all  the  colon  be- 


THE    LARGE    INTESTINE  101 

yond  this  point  may  be  freed  of  its  contents.  If  this  hap- 
pens once  in  twenty-four  hours  the  age  of  the  food  residues 
discharged  will  evidently  vary  from  thirty-three  hours,  for 
the  matter  longest  retained,  down  to  nine  hours,  in  the  case 
of  that  which  barely  came  within  the  section  evacuated. 
There  must  be  extremely  wide  departures  from  these  fig- 
ures in  individual  subjects. 

The  descending  colon  is  reported  to  be  found  empty,  as  a 
rule,  when  observed  with  the  a;-ray.  This  is  taken  to  mean 
that  it  is  an  irritable  segment  which  is  stimulated  to  ad- 
vance all  that  enters  it  to  the  sigmoid  flexure  without  de- 
lay. The  sigmoid  flexure,  on  the  contrary,  permits  a  rela- 
tively large  accumulation  before  it  is  excited  to  contract. 
This  region  of  the  colon  is  interesting  from  a  biologic  stand- 
point, because  it  seems  to  be  an  adaptation  to  the  erect  posi- 
tion of  the  body.  Quadrupeds  hold  the  large  intestine, 
roughly  speaking,  in  a  horizontal  plane,  and  with  them 
the  effect  of  gravity  on  the  contents  is  immaterial.  These 
animals  have  no  sigmoid  flexure,  the  descending  colon  in- 
clining toward  the  midline,  and  joining  the  rectum  without 
the  S-shaped  connection.  When  the  erect  position  is  as- 
sumed there  will  naturally  be  a  tendency  on  the  part  of  the 
fecal  material  to  settle  toward  the  anus.  The  develop- 
ment of  the  sigmoid  in  the  apes  and  in  man  provides  a 
place  of  lodgment  for  the  burden  and  spares  the  rectum 
from  a  constant  distention. 

The  sharp  bend  between  the  sigmoid  and  the  rectum, 
amounting  almost  to  a  kink,  such  as  one  may  see  in  a  rub- 
ber tube  that  has  been  doubled,  is  not  easily  passed  by  the 
feces.  It  is  only  when  the  quantity  has  become  consider- 
able that  a  vigorous  peristalsis  overcomes  the  resistance 
and  fills  the  rectum.  Here  for  the  first  time  the  pressure 
arouses  distinct  sensations.  If  defecation  is  postponed  the 
tone  of  the  rectum  may  be  lowered  and  these  sensations 
cease  to  be  felt.  If  the  occasion  favors,  the  anal  sphincters 
are  inhibited  and  the  rectum  is  emptied  by  its  own  peris- 
talsis, reinforced  by  external  pressure  developed  through 
contractions  of  the  abdominal  muscles  and  the  diaphragm. 


102  NUTRITIONAL   PHYSIOLOGY 

The  action  of  the  skeletal  muscles  at  this  time  resembles 
that  in  vomiting,  but  is,  of  course,  much  more  largely 
voluntary  and  more  sustained  in  character. 

The  Feces. — Two  classes  of  material  may  be  mingled  in 
the  contents  of  the  colon:  the  residues  of  the  diet  and  the 
excretions  of  the  alimentary  tract  including  its  glands. 
The  proportion  existing  between  these  two  is  a  variable 
one.  Physiologists  have  lately  come  to  regard  the  food 
residues  in  the  feces,  under  average  conditions,  as  forming 
a  less  prominent  element  than  was  formerly  supposed. 
A  correspondingly  increased  importance  is  assumed  by  the 
excretions.  The  feces  passed  during  long  fasting  are  evi- 
dently made  up  exclusively  of  bodies  of  the  second  class. 
Such  feces,  while  small  in  amount,  may  have  practically 
the  same  composition  as  those  formed  upon  a  moderate 
and  digestible  diet.  This  suggests  that  the  increased 
quantity  which  results  from  eating  does  not  imply  a  greater 
residue  so  much  as  a  greater  production  of  secretions  along 
the  active  canal.  A  mass  very  much  like  normal  feces 
may  gather  in  an  isolated  loop  of  the  intestine  to  which  no 
food  is  admitted.  Among  the  substances  originating  from 
the  tract  rather  than  from  the  food  are  the  modified  bile- 
pigments,  cholesterin  or  its  derivatives,  mucus,  and  de- 
tached cells. 

Bacteria,  living  and  dead,  intimately  mixed  with  the 
numerous  products  of  their  own  life  activity,  are  promi- 
nent in  the  bowel  discharges.  These  organisms  can  in  one 
sense  be  said  to  have  their  origin  in  the  diet,  since  that  was 
the  source  of  the  primary  seeding  or  infection.  From 
another  point  of  view  they  are  largely  developed  at  the 
expense  of  the  body  itself,  for  they  have  multiplied  in  the 
intestine  and  may  have  lived  in  part  upon  its  secretions  as 
well  as  upon  the  food.  The  gases  of  the  colon  are  due  to 
them. 

True  food  residues  include  the  indigestible  matter  of  the 
diet  and  some  undigested  food — normally  but  little  of  the 
latter.  Absorption  is  strikingly  efficient  with  most  sub- 
jects and  reasonable  diets.     Not  more  than  5  per  cent,  of 


THE    LARGE    INTESTINE  103 

most  foods  goes  to  waste.  The  principal  indigestible  com- 
ponent is  the  cellulose  of  vegetable  tissues.  When  it  is 
abundant,  as  in  a  diet  containing  coarse,  woody  matter, 
such  as  is  eaten  by  the  rabbit,  the  feces  are  given  an  im- 
mense bulk.  The  same  result  is  secured  in  a  measure 
when  man  adopts  a  ration  in  which  fruits  and  vegetables 
enter  freely.  Much  cellulose  is  an  impediment  to  com- 
plete digestion  and  absorption  of  the  proteins  and  carbo- 
hydrates which  it  envelops,  but  the  foods  in  which  it 
occurs  are  cheap  and  the  waste  involved  does  not  open 
an  economic  question  of  moment.  Some  bacterial  decom- 
position of  cellulose  is  said  to  take  place  in  the  colon  and 
may  be  of  slight  advantage  to  us,  not  that  we  profit  by  the 
products  of  such  fermentation,  but  that  a  freer  exposure 
of  proteins  and  starch  may  be  insured. 

Most  writers  have  held  that  a  moderate  amount  of  in- 
digestible material  is  a  desirable  feature  of  the  diet.  This 
teaching  was  emphasized  by  the  celebrated  Sylvester 
Graham  early  in  the  last  century.  Lowell  has  referred  to 
him  as  an  ' 'apostle  of  bran."  Later  authorities  have  not 
recommended  such  wholesale  loading  of  the  tract  with 
husks  and  fibers,  but  an  admixture  of  such  elements  has 
been  generally  advocated.  The  favorable  effects  include 
the  well-recognized  stimulation  of  peristalsis  and  probably 
a  better  distribution  of  food  along  the  canal.  The  indi- 
gestible fraction  of  the  food  has  been  spoken  of  as  ballast 
and  also  by  the  expressive  name  of  ''roughage." 

Under  abnormal  circumstances  the  loss  of  valuable  food 
through  the  stools  may  become  extensive.  This  will  be 
the  case  when  absorption  is  interfered  with  either  by  an 
acceleration  of  peristalsis  or  in  other  ways.  Different 
cathartics  bring  this  about  in  a  varying  manner,  some  by 
hastening  the  rate  of  propulsion,  and  others,  especially  the 
salines,  giving  the  intestinal  contents  a  concentration  and 
a  chemical  character  which  forbids  absorption.  A  mild 
disturbance  of  this  nature  is  more  apt  to  result  in  a  waste 
of  fats  and  fatty  acids  than  of  other  forms  of  food. 

Rectal  Feeding. — While  the  large  intestine  of  man  is  not 


104  NUTRITIONAL   PHYSIOLOGY 

called  upon  in  normal  conditions  to  absorb  much  nutriment, 
it  has  a  marked  reserve  power  to  do  so.  This  is  often  of 
the  greatest  value  in  sickness,  when  it  may  be  possible 
to  tide  over  a  critical  time  and  to  keep  up  a  measure  of 
strength  by  introducing  suitable  foods  into  the  colon. 
Fluid  mixtures  used  in  this  way  must  be  of  low  osmotic 
pressure,  a  fact  which,  unfortunately,  rules  out  the  sugars 
excepting  in  small  amounts.  Milk  and  eggs  are  much  em- 
ployed. Experience  has  shown  that  the  body  makes  a 
partial  use  of  the  proteins  offered,  even  though  they  have 
not  undergone  digestion.  The  results  are  better  when  arti- 
ficial digestion  has  been  brought  about  in  advance.  So  far 
as  we  know  there  is  no  flow  of  efficient  digestive  secretions 
in  response  to  the  introduction  of  food  through  the  rectum. 
It  is  believed  that  nutritive  enemata  may  in  part  at  least 
enter  the  region  of  prevailing  antiperistalsis,  and  that  this 
must  favor  their  long  retention  and  better  utilization. 
Stimulants  may  be  given  through  the  large  intestine  and 
water  to  allay  thirst. 


CHAPTER  XII 
THE  BLOOD 

The  intestinal  lining  is  a  barrier  between  the  mixture 
of  food  and  secretions  within  the  canal  and  the  blood 
which  flows  through  the  neighboring  vessels.  The  main 
problem  to  be  dealt  with  in  treating  of  absorption  is  the 
transfer  of  portions  of  the  intestinal  contents  to  the  blood. 
We  shall  find  that  a  certain  fraction  of  the  incoming  nutri- 
ment is  carried  for  a  time  in  the  lymph,  but  this  is  destined 
before  long  to  blend  with  the  main  current  of  the  circula- 
tion. It  will  be  for  our  advantage  to  become  somewhat 
familiar  with  the  make-up  and  service  of  the  blood  before 
we  discuss  the  entrance  into  it  of  the  products  of  digestion. 

Blood  is  a  carrier.  Regarding  it  as  such,  we  can  con- 
veniently subdivide  its  functions  according  to  the  classes 
of  material  which  it  conveys.  Most  people  will  think  of  it 
as,  first  of  all,  the  bearer  of  food  to  the  tissues.  This  is, 
indeed,  a  matter  of  prime  importance.  The  food  is  added 
to  the  blood  mainly  as  it  flows  through  the  capillaries  of  the 
wall  of  the  alimentary  canal,  and  taken  from  it  by  the 
various  parts  of  the  living  body  in  proportions  correspond- 
ing with  the  degree  of  the  local  activity.  The  largest  tax 
is  that  levied  by  the  skeletal  muscles.  An  unfailing  sup- 
ply of  oxygen  is  a  need  even  more  urgent  from  moment  to 
moment  than  the  presentation  of  food.  Oxygen  is  added 
to  the  blood  in  the  lungs,  and  is  withdrawn  from  it  as  it 
makes  its  round  through  the  body,  the  quantity  consumed 
in  different  organs  being  an  even  better  measure  of  their 
individual  metabolism  than  is  their  appropriation  of  food. 
Where  oxygen  is  taken  from  the  blood,  carbon  dioxid  is 
returned  to  it.  This  indicates  that  the  blood  is  a  bearer  of 
wastes.    The  excess  of  carbon  dioxid  makes  its  escape  dur- 

105 


106  NUTRITIONAL   PHYSIOLOGY 

ing  the  next  transit  of  the  blood  through  the  lungs.  Other 
waste  substances  are  gathered  by  the  blood  as  it  flows  near 
the  active  cells.  The  major  part  of  these  is  destined  for 
excretion  by  the  kidneys,  and  these  organs  receive  so  large 
a  share  of  blood  that  the  entire  volume  must  come  under 
their  influence  within  a  short  space  of  time.  A  minor 
fraction  of  the  waste  finds  an  outlet  in  the  bile,  the  sweat, 
and  through  the  intestinal  wall. 

It  has  already  been  pointed  out  that  every  organ  has  a 
chemical  constitution  and  a  metabolism  peculiar  to  itself. 
Therefore  each  organ  must  make  certain  demands  upon  the 
blood  not  exactly  duplicated  elsewhere.  What  is  more 
important,  each  organ  gives  rise  to  products  unlike  those 
formed  by  any  other  part.  In  a  number  of  cases  these 
products  can  be  shown  to  have  far-reaching  effects.  Their 
existence  has  been  mentioned  in  Chapter  IV.  Recognizing 
the  large  part  which  they  play  in  the  economy  of  the  organ- 
ism, we  may  state  at  this  point  that  an  essential  service 
of  the  blood  is  the  transportation  of  internal  secretions. 

Another  function  of  the  blood,  and  one  often  overlooked, 
is  that  of  equalizing  the  temperature  of  different  regions. 
One  is  reminded  of  the  arrangement  of  a  heating  system  in 
a  house  where  a  fire  burns  in  a  furnace  and  a  circulation 
of  air,  hot  water,  or  steam  disperses  the  heat  through  the 
rooms  above.  Interruption  of  this  circulation  will  allow 
the  upper  stories  of  the  house  to  cool  off,  while  the  base- 
ment is  overheated.  In  the  living  body  heat  production 
is  a  function  of  all  active  tissues.  The  ancients  believed 
that  it  was  the  particular  duty  of  the  heart.  This  organ  is, 
in  fact,  distinguished  for  its  rapid  evolution  of  energy,  but 
it  must  be  remembered  that  it  is  of  small  bulk.  A  much 
larger  share  of  the  aggregate  heat  production  is  borne  by 
the  skeletal  muscles.  These  are  supplemented  to  some 
extent  by  the  liver  and  the  other  great  glands  of  the  body. 
Blood  passing  through  a  tissue  which  is  undergoing  lively 
metabolism  will  have  a  higher  temperature  as  it  leaves  than 
it  had  when  it  entered.  As  it  flows  on  it  will  communicate 
some  of  this  surplus  heat  to  resting  structures  in  which 


THE    BLOOD  107 

little  or  no  heat  production  is  going  on.  This  is  the  case 
with  the  connective  tissues  and  with  the  skin. 

At  the  surface  the  blood  loses  heat,  at  least  under  all 
conditions  which  can  be  called  normal.  The  cooler  blood 
then  returns  to  the  large  vessels,  merges  with  the  heated 
blood  from  the  muscles  and  glands,  and  brings  the 
temperature  of  the  mixture  to  an  average  which  seldom 
varies  materially.  The  means  by  which  this  standard  tem- 
perature is  maintained  will  be  discussed  at  length  in  a 
later  chapter.  It  will  be  well  to  point  out  even  at  this 
time  that  the  skin  is  subject  to  considerable  changes  of 
temperature,  and  that  our  sense  of  being  warm  or  cold 
depends  entirely  on  the  condition  prevailing  at  the  sur- 
face. If  we  turn  again  to  our  illustration  of  a  house  heated 
by  a  ''circulation"  of  hot  air  or  water,  we  shall  recognize  that 
here,  as  in  the  living  body,  there  is  a  constant  escape  of  heat 
to  the  environment.  The  temperature  of  a  pane  of  glass 
in  a  window  of  the  house  will  be  influenced  both  by  the 
internal  and  the  external  state  of  affairs.  The  glass  may  be 
warmed  as  hot  air  is  wafted  against  it  from  within  or  cooled 
by  a  gust  from  without.  So  the  skin  may  be  warmed  by  a 
waxing  of  the  blood-current  close  beneath  it  or  chilled  by  a 
passing  draft. 

Blood  may  be  described  as  a  red  fluid,  but  inspection 
with  the  microscope  shows  at  a  glance  that  its  redness  is 
not  due  to  a  coloring-matter  in  solution  and  uniformly  dis- 
tributed, but  to  the  presence  of  minute  solid  bodies  in  sus- 
pension. These  are  the  red  corpuscles.  The  liquid  in 
which  they  are  swept  about  is  the  plasma.  The  corpuscles 
make  up  something  less  than  one-half  the  total  volume. 
In  view  of  this,  it  is  surprising  that  the  blood  can  have  such 
a  free-flowing  character  and  find  its  way  through  the  cap- 
illaries so  readily.  One  would  anticipate  that  such  a 
mingling  of  solid  particles  with  fluid  would  result  in  the 
formation  of  a  highly  viscid  mass.  That  the  actual  condi- 
tion is  so  different  must  be  due  to  the  absolute  smoothness 
and  the  great  pliability  of  the  corpuscles. 

The  Red  Corpuscles. — The  individual  corpuscle  is  usu- 


108  NUTRITIONAL   PHYSIOLOGY 

ally  seen  in  the  form  of  a  disk  slightly  hollowed  on  its  sur- 
faces. It  is  about  five  times  as  wide  as  it  is  thick.  A  very 
slight  unbalanced  pressure  acting  from  one  side  will  con- 
vert it  into  a  saucer-  or  even  a  cup-shaped  form.  It  is  re- 
markably elastic,  and  springs  back  into  its  original  shape 
as  soon  as  there  is  no  longer  a  force  acting  to  distort  it. 
Most  cells  of  the  body  are  variable  in  outline  and  in  size, 
but  one  red  corpuscle  is  as  much  like  another  as  though  all 
had  been  made  in  the  same  mold.  The  diameter  does  not 
vary  perceptibly  from  ^^-qq  inch.  This  is  the  figure  for 
human  blood;  there  is  a  standard  size  of  corpuscle  for  each 
animal  species. 

The  red  corpuscles  are  often  spoken  of  as  cells,  but  their 
claim  to  rank  as  such  is  questionable.     Regarding  them 


Fig.  15. — Red  blood-corpuscles.  Several  are  shown  in  different 
positions.  The  hollow  centers  are  evident.  In  one  case  two  cor- 
puscles are  overlapped  to  show  that  they  are  transparent.  They 
tend  to  run  into  piles,  as  shown  at  the  right.  A  saucer-shaped  form 
is  represented. 

from  an  anatomic  standpoint,  they  lack  a  feature  which  is 
reckoned  an  essential  of  the  cell;  namely,  the  nucleus. 
On  the  physiologic  side  there  is  no  good  reason  for  suppos- 
ing them  to  be  alive.  The  fairest  view,  probably,  is  to 
look  upon  each  red  corpuscle  as  a  modified  and,  in  a  sense, 
degenerate  cell.  A  single  substance  has  come  to  constitute 
a  very  large  percentage  of  its  make-up.  This  is  the 
peculiar,  iron-containing  protein,  hemoglobin.  We  shall 
not  be  far  wrong  if  we  consider  the  corpuscle  to  be  a  packet 
of  hemoglobin  moving  passively  at  the  mercy  of  the  cur- 
rent. Its  function  is  clear  and  definite.  Hemoglobin  has 
the  property  of  combining  with  oxygen  whenever  the  gas 
is  freely  present  in  the  surrounding  medium.  It  has  also 
the  property  of  releasing  this  oxygen  when  it  comes  into  a 
situation  where  there  is  a  relative  scarcity  of  the  element. 


THE    BLOOD  109 

The  hemoglobin  of  the  blood,  as  can  be  seen  from  what 
has  been  said,  is  ever  passing  from  one  state  to  another,  as 
it  alternately  adds  oxygen  to  itself  and  parts  with  it. 
Evidently  it  can  be  described  as  existing  in  two  varieties: 
a  form  fully  charged  with  detachable  oxygen  (oxyhemoglo- 
bin), and  a  form  from  which  this  loosely  engaged  oxygen 
has  been  removed  (reduced  hemoglobin) .  The  first  is  the 
prevailing  variety  in  arterial  blood  fresh  from  the  lungs, 
and  it  gives  to  such  blood  a  bright  scarlet  color.  The 
venous  blood,  returning  from  the  tissues,  still  contains  a 
goodly  proportion  of  oxyhemoglobin,  but  mingled  with  it 
there  is  now  a  variable  amount  of  the  reduced  compound. 
Reduced  hemoglobin  is  of  a  dark  color,  and  venous  blood, 
in  consequence,  inclines  toward  a  purple.  The  sharply 
contrasting  red  and  blue  so  commonly  used  in  diagrams  to 
distinguish  arteries  from  veins  greatly  exaggerate  the 
actual  difference. 

The  red  corpuscle,  it  will  now  be  clear,  is  a  bearer  of 
oxygen.  A  service  like  this  must  be  dependent  on  the 
extent  of  surface  exposed  for  the  absorption  and  the  dis- 
charge of  the  gas.  The  disk  is  obviously  more  efficient 
than  a  sphere  would  be,  for  it  has  more  surface  in  pro- 
portion to  its  mass.  Small  corpuscles  likewise  must  be 
quicker  to  load  and  to  unload  than  large  ones,  and  this 
helps  us  to  understand  why  the  largest  corpuscles  in  nature 
are  not  found  in  large  animals,  but  in  those  like  the  frog 
and  the  fish,  which  do  not  have  a  very  intense  metabolism. 
There  is  an  additional  reason  why  small  corpuscles  are 
better  fitted  to  meet  the  demand  for  a  great  oxygen  supply: 
the  capillaries  must  be  of  a  size  to  admit  the  corpuscles, 
and  an  animal  with  large  corpuscles  cannot  have  the 
closely  woven  capillary  net  which  becomes  a  possibility 
when  the  diameter  of  the  corpuscles  is  reduced  and  which 
brings  the  oxygen  into  closer  relations  with  the  cells  which 
require  it. 

The  red  corpuscles  originate,  generally  speaking,  by  the 
transformation  of  cells  in  an  unexpected  locality.  This  is 
the  so-called  ''red  marrow''  which  fills  the  small  spaces  that 


110  NUTRITIONAL    PHYSIOLOGY 

abound  in  the  enlarged  extremities  of  the  long  bones. 
Here  there  is  always  going  on  a  progressive  change  in  the 
composition  of  the  cells,  in  course  of  which  hemoglobin 
replaces  most  of  their  original  substance.  The  cell  nuclei 
are  eventually  lost  and  the  newly  formed  corpuscles  de- 
tach themselves  and  drift  awaj^  in  the  blood-stream.  Their 
hollow  centers  strongly  suggest  the  deficiency  due  to  the 
loss  of  the  nuclei.  It  is  an  interesting  fact  that  when  re- 
covery is  taking  place  after  hemorrhage,  red  corpuscles 
containing  nuclei  are  often  to  be  found  in  samples  of  the 
blood.  This  makes  it  seem  as  though  in  meeting  the  emer- 
gency corpuscles  in  an  incomplete  stage  of  their  devel- 
opment had  been  pressed  into  service. 

The  first  glance  at  a  specimen  of  blood  under  the  micro- 
scope leaves  the  impression  that  the  corpuscles  are  all  of  one 
unvarying  type.  Closer  observation  shows  here  and  there 
among  the  host  of  colored  elements  bodies  of  another  or- 
der, the  white  or  colorless  corpuscles.  There  is  but  one  of 
these  to  500  or  1000  red.  The  white  corpuscles  are  not 
flattened,  but  more  nearly  spheric.  They  are,  however, 
of  no  fixed  form,  and  many  of  them  have  the  property  of 
ameboid  movement.  This  is  good  evidence  that  they  are 
to  be  considered  living,  and  it  can  be  shown  further  that 
they  have  nuclei  and  conform  to  our  conception  of  com- 
plete cells.  Much  that  is  of  interest  is  known  of  them,  and 
the  relation  which  they  bear  to  the  checking  of  bacterial 
infection  is  of  the  utmost  importance.  We  shall  not  enter 
upon  a  discussion  of  this  fascinating  subject. 

The  Plasma. — Aside  from  the  transportation  of  oxygen, 
all  the  chief  functions  of  the  blood  could  apparently  be 
fulfilled  by  the  plasma.  The  standard  foods  of  the  tissues 
are  here.  So  also  in  much  smaller  amounts  are  the  non- 
gaseous wastes.  Carbon  dioxid,  the  most  abundant  of  all 
waste-products,  is  carried  jointly  by  the  plasma  and  the 
corpuscles.  It  is  evident  that  we  must  expect  a  solution 
meeting  such  manifold  requirements  to  be  of  a  highly 
complex  nature.     This  is  eminently  the  case. 

More  than  nine-tenths  of  the  plasma  is  water.     The 


THE    BLOOD  111 

proportion  is  little  increased  by  drinking  and  little  reduced 
by  thirst.  Its  constancy  is  secured  by  kidney  activity 
coupled  with  rapid  exchanges  of  fluid  which  take  place  be- 
tween the  blood  and  the  lymph  and  tissues.  When  a  great 
draft  is  made  upon  the  water  of  the  blood,  as  is  the  case  in 
profuse  sweating,  the  volume  is  restored  by  taking  in  water 
from  outside  the  vessels,  and  this  may  mean  a  marked 
loss  of  body  weight.  The  blood  itself  is  not  likely  to  share 
appreciably  in  such  a  loss.  Its  total  volume  in  the  adult 
of  average  build  has  been  variously  estimated  at  from  3  to 
5  quarts.  The  earlier  calculations  led  to  the  higher  figures; 
4  quarts  (8  pounds  or  -^q  of  the  weight)  is  a  reasonable 
standard  for  us  to  adopt. 

Among  the  substances  in  the  plasma  which  can  be  classed 
under  the  general  head  of  foods,  proteins  take  the  first 
place.  They  form  about  four-fifths  of  the  total  solids. 
This  high  proportion  does  not  correspond  at  all  with  the 
make-up  of  an  average  diet,  in  which,  as  we  have  seen, 
proteins  take  the  second  or  even  the  third  place.  It  is 
usual  to  distinguish  three  varieties  of  protein  in  the  plasma, 
serum-albumin,  serum-globulin,  and  fibrinogen,  but  indica- 
tions are  multiplying  which  support  the  belief  that  the  ac- 
tual number  of  proteins  with  clearly  individual  characters 
is  much  greater.  Our  knowledge  of  these  bodies,  their 
mode  of  origin,  place  of  formation,  and  the  particular 
value  of  each  one  in  the  system,  is  in  a  highly  unsatisfac- 
tory state. 

Carbohydrates,  which  occupy  the  leading  position  in  the 
rations  ordinarily  chosen  by  civilized  man,  are  very  scantily 
represented  in  the  plasma.  The  principal  one  present  is 
the  monosaccharid,  dextrose,  known  also  as  glucose,  or 
grape-sugar.  The  percentage  of  dextrose  figured  for  the 
whole  blood  is  between  0.1  and  0.2.  This  means  from  1 
to  2  grams  of  sugar  in  a  liter  of  blood,  and  limits  the  total 
amount  in  circulation  to  10  grams  at  most.  Such  a  quan- 
tity seems  insignificant  when  it  is  remiembered  that  100 
grams  of  sugar  may  easily  be  formed  in  the  digestion  of  a 
moderate  meal.     The  sugar  is  not  readily  increased  by 


112  NUTRITIONAL    PHYSIOLOGY 

free  feeding  of  carbohydrates,  nor  does  it  noticeably  di- 
minish during  long  fasting.  The  explanation  of  this  sin- 
gular constancy  will  be  given  later. 

Fat,  or  its  derivatives,  is  found  in  the  plasma  in  a  pro- 
portion of  a  similar  order  to  that  of  sugar.  It  is  much  more 
subject  to  variation,  rising  notably  after  a  meal  in  which 
there  was  much  fat.  Plasma  obtained  from  a  dog  at  such 
a  time  exhibit  the  phenomenon  of  developing  a  true  cream, 
the  fat  gathering  at  the  surface.  During  starvation  the 
blood  is  not  necessarily  poor  in  fat,  since  it  is  likely  to 
be  engaged  in  carrying  this  form  of  food  from  places  of 
storage — the  adipose  tissue — to  the  muscles  and  elsewhere 
to  be  oxidized. 

Those  compounds  in  the  plasma  which  we  can  confi- 
dently designate  as  waste-products  occur  only  in  the  small- 
est quantities.  Carbon  dioxid  is,  of  course,  an  exception, 
being  very  abundant.  Among  the  non-gaseous  wastes 
destined  to  be  dealt  with  by  the  kidneys,  urea  is  the  only 
one  which  is  easily  detected.  This  is  the  compound  in 
which  seven-eighths  of  the  nitrogen  is  carried  from  the 
body.  The  fact  that  the  waste  substances  are  kept  down 
to  such  a  low  level  in  the  blood  is  evidence  of  the  remark- 
able efficiency  of  the  kidneys  and  the  supplementary  or-* 
gans  of  excretion.  It  is  also  a  reminder  of  how  rapid  and 
copious  is  the  circulation.  No  portion  of  the  blood  can 
long  escape  passage  through  the  glands  which  have  this 
striking  power  to  hold  it  to  a  standard  composition. 

If  the  mixed  solids  from  a  sample  of  plasma  are  inciner- 
ated, we  have  left  a  small  mass  of  ash  or  mineral  matter 
amounting  to  about  1  per  cent,  of  the  whole  blood.  By 
far  the  largest  component  is  sodium  chlorid,  the  chief  salt 
of  the  diet,  and  the  only  one  which  we  deliberately  add  to 
our  food.  Other  bases  represented  are  potassium,  calcium, 
and  magnesium.  Besides  chlorids,  we  find  carbonates  and 
phosphates,  the  former  having  greatly  involved  relations 
with  the  carbon  dioxid  of  the  blood.  To  the  list  which  has 
been  given  might  be  added  many  minor  constituents,  some 
of  which  are  judged  to  be  present  because  of  certain  prop- 


THE    BLOOD  113 

erties  displayed  by  the  blood  rather  than  because  they  can 
be  chemically  identified.  Illustrations  of  such  are  afforded 
by  the  internal  secretions,  enzymes,  and  the  immune  bodies. 
Individual  peculiarities  of  metabolism,  susceptibility,  and 
resistance  must  depend  on  substances  of  this  obscure 
class. 

Coagulation. — Blood  when  shed  shows  the  familiar 
property  of  clotting.  This  is  a  most  valuable  quality, 
since  it  provides  for  the  automatic  checking  of  ordinary 
bleeding  and  also  forms  a  protective  shield,  the  scab,  be- 
neath which  the  healing  process  may  go  on.  The  im- 
portance of  coagulability  is  emphasized  by  the  rather  fre- 
quent observation  of  cases  in  which  it  is  lacking,  and  in 
which  serious  or  even  fatal  hemorrhages  follow  trifling 
injuries.  The  essence  of  the  process  is  a  chemical  change 
affecting  one  of  the  minor  proteins  of  the  plasma  in  such  a 
way  that  it  passes  into  a  solid  form  and  cements  together 
the  red  corpuscles.  The  original  protein  is  the  one  already 
named  as  fibrinogen.  The  modified  form  after  coagulation 
is  called  fibrin.  The  actual  amount  of  fibrin  is  extremely 
small  (perhaps  2  to  4  parts  in  1000  of  blood),  but  it  is  not 
difficult,  when  we  consider  its  gummy  character,  to  under- 
stand how  it  can  convert  a  liquid  medium  into  a  stiff  jelly 
by  knitting  together  the  suspended  corpuscles. 

The  entirely  natural  impression  that  coagulation  is  the 
result  of  exposure  to  the  air  is  erroneous.  The  matter  has 
been  the  subject  of  the  most  painstaking  studies,  of  which 
we  can  give  only  the  briefest  summary.  The  formation  of 
fibrin  is  an  instance  of  enzyme  action,  and  forcibly  recalls 
the  curdling  of  milk  in  the  stomach  under  the  influence  of 
rennin.  The  resemblance  is  not  complete  in  every  re- 
spect, but  is  still  suggestive.  If  we  are  to  assume  that  an 
enzyme  exists  in  the  blood  at  the  time  of  clotting  and  not 
before,  we  must  estabhsh  its  origin.  Reducing  the  facts 
to  the  barest  essentials,  we  may  make  the  following  state- 
ment :  normal  blood  contains  in  great  numbers  minute  and 
extremely  perishable  bodies  known  as  the  blood-plates. 
These  are  much  smaller  than  the  red  corpuscles.     When 

8 


114 


NUTRITIONAL   PHYSIOLOGY 


the  surroundings  are  normal,  as  is  presumably  the  case 
within  the  vessels,  they  keep  their  integrity,  or  at  least  do 
not  disintegrate  en  masse.  When  they  are  brought  in  con- 
tact with  foreign  surfaces  they  do  undergo  prompt  decom- 
position and,  of  course,  their  constituent  material  is  dis- 
solved in  the  plasma.  Something  derived  from  the  blood- 
plates  sets  in  motion  the  series  of  chemical  reactions  which 
leads  at  last  to  the  perfecting  of  an  enzyme,  thrombin, 


Fig.  16. — The  origin  of  the  lymphatics.  This  is  an  extension 
of  Fig.  3.  The  course  of  the  blood  among  the  cells  is  shown  as  be- 
fore (B-B).  A  detail  of  the  lymphatic  system  is  added  {Ly.). 
The  drawing  is  severely  conventional.  Two  branches  of  the  lym- 
phatic are  represented  as  beginning  with  open  mouths  so  situated  as 
to  receive  the  surplus  fluid  directly  from  the  interstices  of  the  tissue. 
A  third  is  connected  with  a  blind  sac.  There  is  at  present  some  dis- 
agreement as  to  which  is  the  more  typical  mode  of  origin. 


capable  of  transforming  the  soluble  fibrinogen  into  the 
insoluble  fibrin. 

Lymph. — This  term  is  usually  made  to  stand  for  the 
fluid  outside  the  blood-vessels.  Used  in  this  way,  it 
includes  the  liquid  filling  the  microscopic  intervals  between 
neighboring  cells,  the  larger  spaces  which  often  occur  in 
the  loosely  woven  connective  tissues,  and  also  the  contents 
of  an  inconspicuous  system  of  tubular  channels  known  as 
lymphatics.     A    recent   writer.    Starling,    has    suggested 


THE    BLOOD  115 

that  a  distinction  should  be  made  between  the  fluid  in  the 
lymphatics  and  that  which  is  not  enclosed  in  vessels  of  any 
sort.  He  would  restrict  the  term  lymph  to  the  first  appli- 
cation, and  would  speak  of  the  other  as  tissue-fluid.  This 
usage  appears  highly  desirable,  but  is  not  as  yet  widely 
current. 

Lymph,  in  the  sense  of  tissue-fluid,  cannot  be  collected 
for  analysis.  Considering  the  delicacy  of  the  capillary 
wall  and  the  probable  freedom  with  which  exchanges  take 
place  through  it,  there  is  reason  to  believe  that  this  fluid 
must  closely  resemble  the  blood-plasma.  When  a  tissue 
is  the  seat  of  an  active  metabolism  the  income  and  outgo 
of  its  cells  must  tend  to  alter  the  composition  of  the  adja- 
cent lymph  and  to  make  it  less  like  the  plasma.  The 
general  effect  will  be  to  reduce  the  oxygen  to  a  low  level,  to 
raise  the  carbon  dioxid  content  correspondingly,  to  con- 
sume a  fraction  of  the  organic  food,  and  to  add  miscella- 
neous waste-products.  The  lymph  which  can  be  obtained 
by  cutting  one  of  the  larger  lymphatics  of  the  body  has 
these  characters.  This  lymph  may  not  be  precisely  like 
the  mixture  which  exists  in  close  contact  with  the  living 
cells,  but  it  is  probable  that  the  differences  are  not  great. 
According  to  a  view  formerly  universal  and  still  held  by 
many,  lymph  can  work  its  way  from  any  interstice  of  an 
organ  into  the  branches  of  the  lymphatics,  and  so  to  larger 
vessels  of  the  same  class.  Many  believe,  however,  that 
there  is  a  definite  separation  between  the  unwalled  spaces 
of  the  tissues  and  the  interior  of  the  true  lymphatic  system. 
If  this  is  the  correct  conception,  we  have  good  reason  to 
differentiate  lymph  from  tissue-fluid.  We  may  adopt 
either  view  provisionally  without  being  seriously  misled. 
Lymph  contains  white  corpuscles  and  blood-plates. 
Since  it  has  in  it  some  fibrinogen  it  may  coagulate,  but 
owing  to  the  absence  of  red  corpuscles  the  clot  is  frail  and 
tremulous. 


CHAPTER  XIII 
THE   CIRCULATION 

We  must  postpone  still  further  our  account  of  the  ab- 
sorption of  the  products  of  digestion  until  we  shall  have 
made  clear  the  general  course  followed  by  the  circulating 
blood.  We  have  quoted  the  estimate  that  the  body  con- 
tains 8  or  10  pounds  of  blood.  At  a  given  moment  about 
one-fourth  of  this  may  be  assumed  to  be  in  the  thorax 
(the  heart,  the  lungs,  and  the  great  blood-vessels),  a 
fourth  in  the  skeletal  muscles,  a  fourth  in  the  liver,  and  the 
remaining  fourth  elsewhere.  The  ceaseless  movement  of 
this  large  volume  of  liquid  is  maintained  by  the  beating  of 
the  heart. 

This  organ  consists  of  two  halves,  right  and  left,  com- 
pletely separated,  so  far  as  their  cavities  are  concerned,  by 
a  middle  partition.  Regarded  as  a  mass  of  muscle,  the 
heart  is  single;  considered  as  a  pump,  it  is  double.  Each 
half  is,  in  a  literal  sense,  a  force-pump.  Each  side  shows 
us  two  communicating  chambers,  an  auricle  above  and  a 
ventricle  below.  The  auricles  receive  the  blood,  which  the 
corresponding  ventricles  will  shortly  discharge.  The 
vessels  leading  to  the  auricles  are  called  veins ;  those  which 
convey  the  blood  from  the  ventricles  are  called  arteries. 
The  auricles  have  thin  walls,  while  the  ventricles  are 
fitted  for  their  task  by  heavy  muscular  development. 
The  left  ventricle  has  much  more  power  than  its  fellow, 
and  the  necessity  for  this  will  soon  be  evident. 

A  single  great  artery,  the  aorta,  springs  from  the  left 
ventricle.  Its  branches  reach  all  parts  of  the  body.  Sub- 
dividing repeatedly,  they  introduce  the  blood  at  last  into 
the  capillaries,  the  innumerable  vessels  of  the  smallest 
order  through  whose  exquisitely  thin  walls  take  place  the 

116 


THE    CIRCULATION 


117 


exchanges  with  the  l3anph  already  described.  The  word 
'^capillary"  means  hair-like,  but  the  description  falls 
far  short  of  indicating  the  actual  slenderness  of  these  mi- 
croscopic tubes.  They  are  so  narrow  that  the  corpuscles 
pass  through  them  practically  in  single  file.     The  capillar- 


Fig.  17. — In  this  diagram,  as  is  usual  in  such  cases,  right  and  left 
are  reversed.  This  is  as  though  the  observer  were  looking  at  an- 
other subject.  The  short  pulmonary  path  is  to  be  traced  from  the 
right  ventricle  to  the  left  auricle  (P.  C).  Alternative  routes  are 
suggested  for  the  passage  of  the  blood  through  the  greater  circula- 
tion from  the  left  ventricle  to  the  right  auricle.  The  blood  which 
traverses  the  digestive  tract  (D)  passes  through  a  second  set  of 
capillaries  in  the  liver  (L)  before  it  can  return  to  the  heart.  Note 
that  the  liver  has  in  addition  a  separate  arterial  supply  of  blood. 


ies  are  short  and  soon  unite  to  form  the  smallest  veins. 
These  lead  the  blood  back  toward  the  heart,  joining  as 
they  go  to  form  larger  and  less  numerous  channels,  until  at 
the  last  there  are  but  two  great  veins.  These  enter  the 
right  auricle,  one  from  above  and  one  from  below.  The 
sweep  of  the  blood  from  the  left  ventricle  through  the  body 


118  NUTRITIONAL   PHYSIOLOGY 

at  large  and  back  to  the  right  auricle  is  called  the  systemic 
circulation. 

From  the  right  auricle  the  blood  descends  to  the  ven- 
tricle of  the  same  side,  and  is  sent  forth  again,  this  time 
through  the  vessel  known  as  the  pulmonary  artery. 
This  immediately  forks  into  branches  which  plunge  into  the 
two  lungs.  The  smaller  pulmonary  arteries  lead  to  rich 
capillary  networks  which  are  wrapped  around  the  number- 
less air-sacs  of  the  lungs.  Here  the  corpuscles  are  re- 
charged with  oxygen  and  the  carbon  dioxid  of  the  blood  is 
reduced  to  a  standard  amount.  Pulmonary  veins  return 
the  blood  to  the  left  auricle.  The  relatively  short  journey 
of  the  blood  from  the  right  ventricle  to  the  left  auricle 
by  way  of  the  lungs  is  called  the  pulmonary  or  lesser  cir- 
culation. 

It  is  necessary  to  call  attention  to  the  fact  that  the  ad- 
jectives ^ ^arterial"  and  'Venous"  are  not  used  in  a  sense 
which  exactly  corresponds  with  the  meanings  of  the  nouns 
' 'artery"  and  'Vein."  The  adjectives  have  a  chemical 
significance;  the  nouns,  an  anatomic  one.  Arterial  blood 
is  blood  fully  oxygenated;  venous  blood  is  blood  more  or 
less  deficient  in  oxygen.  But  an  artery,  as  has  been  said, 
is  merely  a  vessel  carrying  blood  away  from  the  heart. 
The  blood  within  will  be  arterial  if  we  are  observing  the 
systemic  circulation,  and  venous  if  it  is  in  the  pulmonary. 
So  the  systemic  veins  are  filled  with  venous  blood,  while  the 
pulmonary  veins  carry  that  which  has  just  been  brought 
up  to  the  arterial  standard  through  coming  into  relation 
with  the  air  in  the  lungs. 

From  what  was  stated  above  with  respect  to  the  distri- 
bution of  the  blood,  it  is  evident  that  less  than  one-fourth 
of  the  whole  volume  is  in  the  pulmonary  circulation  at  one 
time,  but  the  student  must  be  cautioned  against  the  con- 
clusion that  the  pulmonary  circuit  is  traversed  by  less 
blood  than  passes  through  the  systemic  pathways.  The 
quantities  passing  along  the  two  routes  are  inevitably 
equal,  for  it  is,  after  all,  the  same  blood  which  runs  through 
each  in  turn.     If  a  chain  is  traveling  over  two  wheels,  as 


THE    CIRCULATION  119 

shown  in  the  diagram  (Fig.  18),  the  links  pass  the  two  in 
equal  numbers,  and  yet  there  are  constantly  more  links  on 
their  way  from  L  to  R  than  from  R  to  L. 

When  we  speak  of  a  heart-beat  we  mean  a  co-ordinated 
contraction  of  the  heart's  peculiar  muscle.  The  phe- 
nomenon includes  two  phases:  a  brief  and  not  forcible 
contraction  of  the  auricles  followed  by  a  longer  and  much 
more  powerful  closing  in  of  the  ventricles  upon  their  cavi- 
ties. The  ventricular  contraction  is  the  essential  factor  in 
propelling  the  blood.  Valves  between  the  auricles  and  the 
ventricles  permit  the  latter  to  fill  during  their  period  of 


Fig.  18. — For  explanation  of  figure  see  text. 

relaxation,  and  forbid  the  backward  flow  from  the  ventricles 
when  they  are  emptying  themselves.  Other  valves  at  the 
commencement  of  the  aorta  and  the  pulmonary  artery 
allow  blood  to  pass  out  during  each  contraction  of  the 
ventricles,  but  not  to  return  from  either  artery  into  the 
heart  when  the  resting  phase  ensues.  The  action  of  the 
two  sets  of  valves  is  precisely  on  the  principle  of  the  two 
which  make  a  syringe-bulb  an  effective  force-pump.  The 
right  and  left  halves  of  the  heart  act  practically  at  the 
same  time. 

Either  ventricle  vv^hen  full  may  contain  5  or  6  fluidounces 
of  blood.     As  it  contracts  it  reduces  its  capacity  and  dis- 


120  NUTRITIONAL    PHYSIOLOGY 

charges  blood  in  like  measure.  At  the  conclusion  of  the 
active  period  it  may  have  nearly  obliterated  its  cavity,  or 
the  effort  may  fail  while  there  is  still  considerable  blood 
within.  Estimating  an  average  output  from  one  ventricle 
to  be  about  4  ounces,  or  100  c.c,  it  is  apparent  that  forty 
beats  will  suffice  to  discharge  from  one  side  of  the  heart 
an  amount  of  blood  equivalent  to  the  entire  quantity  of 
circulation  (40  x  100  =  4000  c.c,  or  4  liters).  In  other 
words,  it  will  require  less  than  one  minute  for  all  the  blood 
to  pass  successively  through  both  sides  of  the  heart.  When 
this  statement  is  made  it  should  be  remembered  that 
certain  routes  in  the  systemic  circuit  are  many  times  longer 
than  others  (compare  the  path  to  and  from  the  feet  with 
that  to  and  from  the  esophagus),  and  some  corpuscles  may 
revisit  the  heart  two  or  three  times  while  others  are  making 
one  prolonged  journey. 

It  would  be  out  of  place  to  enter  here  upon  a  discussion 
of  the  value  of  the  auricles.  From  a  mechanical  point  of 
view  they  contribute  but  little  of  the  energy  required  for 
driving  the  blood.  They  serve  to  accommodate  the 
gathering  volume  of  blood  which  the  veins  bring  in  during 
the  rather  prolonged  contraction  of  the  ventricles,  and  they 
secure  a  more  efficient  filling  of  the  lower  chambers  of  the 
heart.  While  their  muscular  development  is  sHght,  their 
automatic  power  is  peculiarly  marked,  and  it  is  a  fair 
statement  that  the  ventricle,  under  normal  conditions,  is 
stimulated  to  perform  each  beat  by  an  influence  radiating 
to  it  from  the  auricle.  If  there  is  an  interruption  of  certain 
strands  of  tissue  which  normally  unite  the  two,  they  cease 
to  maintain  the  same  rhythm,  the  auricle  continuing  at 
the  accustomed  rate,  while  the  ventricle  beats  much  less 
frequently.  When  the  heart  is  dying  the  last  signs  of 
pulsation  are  in  the  auricles. 

The  Portal  Circulation. — In  our  general  description  of 
the  course  followed  by  the  blood  on  its  way  through  the 
body  it  was  stated  that  when  it  has  passed  through  a  set 
of  capillaries  it  is  gathered  up  into  veins  to  be  returned 
to  the  right  auricle.     An  important  departure  from  this 


THE    CIRCULATION  121 

order  is  now  to  be  indicated.  The  blood  which  has  trav- 
ersed the  small  vessels  of  the  digestive  tract  (including  the 
stomach,  both  intestines,  the  pancreas,  and  the  spleen) 
w^ould  be  expected  to  make  its  way  back  to  the  heart 
through  branches  of  the  systemic  veins.  The  actual  ar- 
rangement is  as  follows:  A  large  vessel  receives  all  the 
blood  from  this  region  and  carries  it  into  the  liver,  where 
a  second  set  of  capillaries  is  entered.  When  these  unite 
it  is  to  form  short  veins  discharging  into  the  chief  vein  of  the 
body,  just  below  the  diaphragm  and  practically  within  the 
boundaries  of  the  liver.  The  channel  leading  from  the 
organs  of  digestion  to  the  liver  occupies  a  unique  position. 
Inasmuch  as  it  is  formed  from  a  capillary  system  it  seems 
to  be  a  vein,  but  since  it  also  supplies  a  capillary  system 
it  may  be  contended  that  it  is  an  artery.  Its  structure 
favors  the  view  that  it  is  a  vein  and  it  is  so  called.  The 
vessels  which  gather  the  blood  from  the  digestive  tract 
and  distribute  it  inside  the  liver  are  said  to  form  the  portal 
system;  the  chief  conductor  described  above  is  called  the 
portal  vein.  An  important  consequence  of  this  arrange- 
ment is  that  the  absorbed  products  of  digestion,  so  far  as 
they  are  in  the  blood  rather  than  the  lymph,  are  brought 
under  the  influence  of  the  liver  before  they  go  elsewhere. 

The  question  will  naturally  be  raised  whether  the  liver 
is  supplied  exclusively  with  venous  blood.  Provision  is 
made  for  a  supplementary  supply  through  what  is  known 
as  the  hepatic  artery,  a  vessel  which  is  an  offshoot  of  the 
general  arterial  system,  and  which,  of  course,  introduces 
into  the  liver  blood  rich  in  oxygen.  An  analogous  condi- 
tion may  be  noted  in  the  lungs,  where  the  main  currents 
are  the  venous  streams  coming  from  the  right  side  of  the 
heart,  but  where  there  are  also  interwoven  in  the  tissue 
much  smaller  vessels  which  take  their  rise  in  the  arterial 
tree  springing  from  the  left  ventricle. 

The  problems  of  the  circulation  fall  into  two  classes: 
There  are  those  which  are  purely  physical  and  which  can 
be  approximately  reproduced  and  more  or  less  successfully 
studied  in  lifeless  models.     There  are  also  those  which  have 


122  NUTRITIONAL    PHYSIOLOGY 

to  do  with  the  behavior  of  the  organs  concerned  when  they 
are  vieAved  as  living  structures.  Under  the  first  class  are 
included  the  facts  of  blood-pressure,  velocity  of  flow,  the 
resistance  overcome,  etc.  The  interpretation  of  such  data 
is  almost  a  science  in  itself  and  is  known  as  hemodynamics. 
Among  the  questions  of  the  second  sort  are  the  mysterious 
automaticity  of  the  heart  and  the  nervous  government  of 
the  quantity  and  the  distribution  of  blood  flow.  These 
difficult  matters  may  well  be  left  to  be  briefly  dealt  with 
toward  the  end  of  the  book,  when  the  general  work  of  the 
central  nervous  system  will  be  presented.  We  must, 
however,  devote  some  space  at  this  time  to  the  fundamen- 
tals of  hemodynamics. 

The  Character  of  the  Blood  Flow  in  Vessels  of  DiiBferent 
Classes. — When  an  artery  is  cut  the  blood  escapes  in  a 
forcible  jet  which  may  spring  to  a  distance  of  several  feet. 
The  stream  is  not  steady,  but  mounts  and  declines  in  a 
rhythm  corresponding  with  the  heart-beat.  A  good  deal 
of  pressure  must  be  applied  to  restrain  the  bleeding.  If  a 
vein  is  severed  the  flow  of  blood  is  rapid  and  copious,  but 
easily  repressed.  It  is  uniform  so  far  as  can  be  judged. 
These  observations  lead  to  the  conclusion  that  the  blood 
in  the  arteries  is  under  a  high  average  pressure,  with  large 
fluctuations  from  moment  to  moment.  The  pressure  in 
the  veins  is  evidently  very  low. 

The  diminution  of  pressure  between  the  arteries  and  the 
veins  can  be  simply  explained.  When  the  blood  is  started 
on  its  course  through  the  systemic  circulation  the  high 
pressure  which  it  exerts  against  the  elastic  wall  may  be 
regarded  as  a  measure  of  the  energy  which  the  ventricle 
has  impressed  upon  it.  When  it  draws  near  the  right  side 
of  the  heart,  the  goal  of  its  journey,  its  abated  pressure  is 
the  sign  that  the  initial  energy  has  been  spent.  How  has 
it  been  consumed?  There  is  but  one  possible  answer:  It 
has  been  transformed  into  heat  in  overcoming  the  friction 
encountered  in  the  vessels.  Analogous  conditions  can  be 
demonstrated  for  any  tubular  system  through  which  liquid 
is  driven.     In  the  mains  which  carry  a  city  water-supply 


THE    CIRCULATION  123 

from  a  pumping-station  to  distant  points  there  is  a  similar 
decline  in  pressure  along  each  line  as  it  is  followed  farther 
from  the  fountain  head.  (The  assumption  is  made  that 
we  have  to  do  with  pipes  which  are  on  the  same  level 
throughout.) 

In  any  such  system  the  cutting  down  of  the  pressure  will 
be  abrupt  at  any  point  on  the  route  where  there  is  an 
unusual  impediment  to  the  flow.  This  is  the  case  in  the 
body  where  the  blood-steam  is  so  extensively  subdivided 
to  enter  the  smallest  vessels.  Subdivision  of  channel 
means  multiplied  surface  for  friction.  Thus  there  occurs 
a  radical  drop  in  pressure  between  the  smallest  arteries  in 
which  we  can  measure  it  and  the  veins  of  similar  size. 
The  highest  pressure  developed  anywhere  must  be  in  the 
left  ventricle  when  it  is  discharging  the  blood,  for  it  is 
then  to  be  regarded  as  the  starting-point  of  the  circulation. 
The  lowest  pressure  which  is  ever  registered  is  probably  in 
the  same  ventricle  when  it  is  beginning  to  fill,  for  now  it  is 
the  terminus  of  one  of  the  two  circuits. 

The  swelling  of  the  arteries  which  promptly  follows  each 
ventricular  contraction  is  what  we  recognize  as  the  pulse. 
It  is  the  sign  of  a  new^ly  introduced  portion  of  blood  dis- 
tributing itself  in  the  vessels.  We  have  said  that  the  veins 
show  no  such  rhythmic  enlargement.  We  have,  therefore, 
to  account  for  the  conversion  of  the  intermittent  flow  in  the 
arteries  into  the  constant  flow  in  the  veins.  The  principal 
factor  concerned  is  the  marked  elasticity  of  the  arterial 
trunks.  It  will  be  helpful  to  refer  to  the  artificial  devices 
which  serve  the  same  purpose  in  connection  with  force- 
pumps.  A  familiar  one  is  the  air-chamber.  This  is  a 
large  container  communicating  with  the  outflow-pipe. 
Whenever  a  stroke  of  the  pump  drives  water  along  this 
pipe  a  certain  share  proceeds  directly  toward  the  outlet, 
while  another  fraction  turns  aside  into  the  air-chamber. 
During  the  return  stroke  when  no  water  is  issuing  from  the 
pump  barrel  the  air  which  a  moment  before  underwent 
compression  in  the  chamber  tends  to  regain  its  original 
volume,  and  in  so  doing  forces  water  through  the  lateral 


124  NUTRITIONAL   PHYSIOLOGY 

branch  and  thence  along  the  main  pipe.  The  intermittent 
dehvery  through  the  valve  is  transformed  more  or  less 
successfully  into  a  continuous  flow  through  the  remote 
outlet. 

The  neutralizing  of  intermittency  in  the  blood  system  is 
referable  to  the  same  principle,  but  the  elastic  compensator 
is  not  to  be  found  as  a  single  locahzed  feature;  it  is  discov- 
ered in  the  universal  capacity  of  the  arteries  to  stretch  and 
to  regain  their  former  size.  At  the  beginning  of  the  aorta 
or  of  the  pulmonary  artery  we  have  complete  intermittency, 
the  blood  alternately  forging  ahead  and  halting.  But  each 
portion  of  blood  ejected  from  the  heart  finds  room  for 
itself  partly  by  distending  the  arteries  and  not  altogether 
by  driving  forward  the  blood  which  is  before  it.  Hence, 
when  the  ventricles  relax  and  the  outpouring  •  of  blood 
ceases  there  is  still  an  onward  movement  in  the  smaller  and 
more  distant  arteries,  because  the  larger  ones,  which  were 
momentarily  overdistended,  are  now  contracting  and  send- 
ing along  part  of  their  accumulation.  The  farther  we  go 
from  the  heart  the  more  largely  the  driving  of  the  blood  is 
to  be  attributed  to  the  reaction  of  the  stretched  arterial 
walls  and  the  more  nearly  uniform  it  becomes.  This  does 
not  mean  that  the  whole  power  keeping  up  the  circulation 
is  not  to  be  sought  in  the  heart-beat;  it  merely  means  that 
this  energy  may  be  stored  temporarily  by  these  elastic 
structures  and  rendered  back  again. 

The  facts  we  have  been  treating  may  be  expressed  in 
another  way.  The  arterial  tree  forms  a  reservoir  of  con- 
siderable capacity.  Within  it  is  an  amount  of  blood  so 
large  that  the  single  contribution  of  the  ventricle  makes 
a  rather  small  addition  to  it.  The  escape  of  the  blood 
through  the  terminal  twigs  cannot  cease  while  there  is 
so  much  stored  under  a  high  pressure  in  the  aorta  and  its 
branches.  The  heart  may  omit  or  ''drop"  a  beat  without 
noticeably  diminishing  the  flow  through  the  capillaries. 
A  standstill  will  be  reached  only  when  the  arteries  have 
attained  a  degree  of  contraction  such  that  the  internal 
pressure  is  no  higher  than  that  in  the  veins.     The  homely 


THE   CIRCULATION 


125 


illustration  (Fig.  19)  which  accompanies  this  may  be  help- 
ful. The  pump  delivers  water  intermittently  to  the  leaky 
trough,  keeping  it  filled  to  a  level  which  is  nearly  constant, 
though  fluctuating  a  little  in  the  rhythm  of  the  strokes. 
Meanwhile  the  escape  of  water  through  the  cracks  is 
all  but  uniform  in  its  rate.  One  important  difference  be- 
tween the  pump  and  trough,  on  the  one  hand,  and  the  cir- 
culatory system,  on  the  other,  lies  in  the  fact  that  in  the 


Fig.  19. — At  /  the  discharge  is  in  gushes  with  pauses  between — 
the  type  of  the  expulsion  of  blood  from  the  heart.  At  C  the  escape 
through  the  cracks  is  at  a  practically  constant  rate ;  this  is  true  of  the 
blood  flow  through  the  capillaries. 


first  case  the  driving  force  is  gravity;  in  the  second,  it  is 
the  reaction  of  the  enclosing  elastic  walls. 

A  set  of  facts  which  it  is  well  to  separate  in  one's  thought 
as  completely  as  possible  from  considerations  of  pressure 
is  the  body  of  data  respecting  the  linear  velocity  of  the 
blood.  By  this  is  meant  the  rate  of  advance  of  the  aver- 
age corpuscle.  In  any  vessel  the  stream  runs  more  swiftly 
in  the  central  axis  and  lags  along  the  walls.  The  velocity 
in  the  arteries  rises  and  falls  as  does  the  pressure,  but,  on 


126  NUTRITIONAL   PHYSIOLOGY 

the  whole,  is  relatively  high.  The  aorta  is  passed  at  a 
speed  of  at  least  a  foot  in  a  second.  The  veins  also  are 
traversed  at  a  high  velocity,  though  the  figures  are  some- 
what lower  than  for  the  arteries.  The  movement  in  the 
capillaries  is  in  sharp  contrast  to  that  in  both  arteries  and 
veins,  being  exceedingly  slow,  perhaps  jV  i^^h  in  a  second. 
It  appears  that  a  corpuscle  may  take  as  long  to  go  through 
a  capillary  link  which  would  scarcely  span  the  breadth  of  a 
pin-head  as  to  travel  from  the  heart  to  the  brain. 

When  it  is  remembered  that  the  actual  service  of  the 
blood  to  the  tissues  is  rendered  in  the  capillaries  (since  all 
other  vessels  have  walls  too  thick  to  permit  free  diffusion), 
the  value  of  the  slow  passage  is  obvious.  At  the  same  time, 
one  recognizes  the  desirability  of  the  rapid  transit  to  and 
from  this  department  of  the  system.  The  heart  itself  and 
all  the  main  vessels  may  be  thought  of  as  accessory  to  the 
capillaries.  The  explanation  of  the  slowing  of  the  stream 
in  the  small  channels  is  entirely  simple,  yet  often  mis- 
apprehended. Whenever  an  artery  forks  to  form  two 
branches,  these  are  individually  of  smaller  cross-section 
than  the  parent  stem,  but  their  combined  cross-section  is 
greater.  The  result  is  the  same  that  is  seen  when  a  river 
widens  or  deepens — the  current  slackens.  If  the  river 
broadens  into  a  lake  the  current  may  become  impercep- 
tible, yet  we  know  that  the  water  is  still  setting  forward 
toward  the  outlet. 

Are  we  then  to  believe  that  the  capillary  system  is  many 
times  wider  than  the  aorta  or  the  great  veins?  There  is  no 
escape  from  this  conclusion:  their  number  is  so  vast  that, 
despite  their  infinitesimal  size  as  single  conveyors  of  the 
blood,  collectively  they  form  the  broadest  division  of  the 
entire  path.  If  they  fail  to  suggest  a  lake,  an  analogy  may 
be  found  in  the  tangled  swamp  in  which  a  stream  loses 
itself,  breaking  into  many  sluggish  arms,  from  which  at 
last  the  waters  converge  to  resume  a  rapid  course  over  a 
narrow  bed.  The  acceleration  noticed  as  the  veins  are  fol- 
lowed toward  the  heart  is  merely  a  sign  that  as  their  num- 
ber grows  less  their  combined  cross-section  also  contracts. 


THE    CIRCULATION  127 

The  Movement  of  the  Lymph. — Just  as  we  find  veins 
returning  from  every  part  of  the  body,  we  can  make  out, 
though  with  much  greater  difficulty,  small  vessels  bringing 
lymph  in  the  same  general  direction,  that  is,  toward  the 
thorax.  Like  the  veins,  they  unite  as  they  draw  near  the 
heart,  and  the  great  majority  eventually  contribute  to  a 
trunk  known  as  the  thoracic  duct.  This  springs  from  the 
union  of  many  branches  in  the  abdominal  cavity,  pierces 
the  diaphragm,  and  can  be  traced  upward  in  front  of 
the  spinal  column  until  it  empties  into  a  great  vein  which 
is  bringing  the  blood  from  the  left  shoulder  toward  the 
right  auricle.  A  comparatively  insignificant  group  of 
lymphatics  centers  at  an  outlet  in  the  corresponding  posi- 
tion on  the  right. 

The  onward  movement  of  the  lymph  in  its  channels  is 
parallel  with  that  of  the  venous  blood,  but  is  incomparably 
slower.  It  is  sometimes  almost  entirely  arrested.  In  the 
last  chapter  it  was  stated  that  we  cannot  confidently  say 
whether  the  lymphatics  drain  all  the  microscopic  spaces 
in  the  tissues  or  whether  they  rise  in  small  definite  enclos- 
ures. In  either  case  they  are  bearing  away  a  certain  over- 
flow of  fluid  and  may  be  regarded  as  supplementing  the 
service  of  the  veins.  As  to  the  cause  of  the  halting  move- 
ment of  their  contents,  the  simplest  statement  that  can 
be  made  is  somewhat  as  follows:  The  formation  of  new 
lymph  in  all  the  organs  crowds  away  the  lymph  previously 
in  the  beginnings  of  the  lymphatics,  and  this  is  the  central 
fact  to  be  considered.  The  energy  required  is  derived 
partly  from  the  heart,  since  liquid  may  be  forced  out  of 
the  capillaries  by  its  transmitted  pressure,  and  partly  from 
other  sources  too  obscure  to  be  discussed.  The  lymph 
is  generally  referred  to  as  a  carrier  of  waste,  but  a  partial 
exception  must  be  made  in  favor  of  the  lymph  coming 
from  the  intestinal  area  during  digestion.  This  lymph 
may  contain  absorbed  food,  principally  fat.  It  was  in  the 
mesentery  that  lymphatics  distended  with  milky  liquid 
were  first  seen.  They  were  called  lacteals  because  of  their 
appearance,  and  this  term  is  still  used  in  a  local  sense. 


CHAPTER  XIV 
THE  ABSORPTION   OF  THE  FOOD-STUFFS 

If  two  unlike  solutions  are  separated  by  a  membrane, 
such  as  a  sheet  of  parchment  or  some  artificial  substitute, 
they  will  usually  tend  to  equalize  both  in  composition  and 
concentration.  When,  for  example,  potassium  chlorid  and 
sodium  chlorid  solutions  are  placed  on  opposite  sides  of 
such  a  partition,  each  salt  proves  its  ability  to  pass  through 
the  barrier,  and  in  the  course  of  time  there  will  be  uniform 
mixtures  in  both  compartments.  This  is  said  to  show  that 
the  salts  are  diffusible  and  that  the  membrane  is  permeable. 
Different  membranes  are  permeable  in  very  different 
degrees,  and  the  freedom  with  which  various  salts  pass 
through  a  particular  membrane  is  also  far  from  constant. 
Some  substances  may  appear  freely  soluble  and  may  go 
readily  through  ordinary  filters,  but  may  hardly  diffuse  at 
alL  This,  as  a  rule,  is  the  case  with  the  proteins.  If  a 
mixture  of  unboiled  white  of  egg  and  sodium  chlorid  is  on 
one  side  of  a  membrane  and  the  other  side  is  washed  with 
running  water,  nearly  all  the  salt  will  escape,  leaving  the 
protein  practically  undiminished.  The  process  by  which 
diffusible  salts  are  encouraged  to  separate  themselves  from 
substances  which  cannot  accompany  them  through  the 
membrane  into  the  water  beyond  it  is  called  dialysis. 

The  products  of  digestion  are,  in  general,  much  more 
diffusible  than  the  food-stuffs  from  which  they  are  derived. 
Starch,  even  when  boiled  for  a  long  time,  does  not  make 
its  way  through  ordinary  membranes;  the  sugars  do  so 
with  relative  ease.  Fats  are  not  even  soluble  in  water; 
soaps  and  glycerin  are  diffusible  compounds.  Peptones 
arising  from  the  hydrolysis  of  proteins  have  some  power 
to  penetrate  membranes,  and  the  simpler  amino-acids  pass 

128 


THE    ABSORPTION    OF   THE    FOOD-STUFFS  129 

still  more  freely.  We  might  picture  the  situation  in  the 
intestine  somewhat  as  follows:  The  blood  flows  steadily 
beneath  a  rather  complex  cellular  wall,  the  other  surface 
of  which  is  bathed  by  the  mixed  products  of  digestion  and 
the  digestive  secretions.  Peptones  and  amino-acids,  su- 
gars, soaps,  and  glj^cerin,  being  formed  in  relative  abun- 
dance in  the  intestine,  diffuse  into  the  blood,  which  contains 
little  of  these  bodies.  If  the  blood  were  to  stand  still  the 
small  volume  in  direct  relation  with  the  absorbing  surface 
would  accumulate  digestive  products  until  it  held  them  in 
the  same  concentration  in  which  they  exist  in  the  canal. 
Then  a  state  of  equilibrium  would  be  established  and  no 
further  transfer  to  the  blood  would  occur. 

Detailed  observation  shows  that  the  facts  of  absorption 
cannot  be  expressed  in  this  simple  manner.  It  has  already 
been  hinted  that  large  allowance  must  be  made  in  such  a 
case  for  the  fact  that  the  membrane  under  examination  is 
alive.  Its  cells  may  discharge  material  at  one  surface 
quite  unlike  that  which  they  receive  at  the  other.  They 
probably  have  a  considerable  metabolism.  This  means 
that  energy  is  set  free  within  their  borders  and  a  share  of  it 
may  be  applied  to  the  moving  of  the  absorbed  food.  We 
have  previously  called  attention  to  the  parallelism  be- 
tween secretion  and  absorption. 

Before  we  can  go  further  with  this  discussion  something 
must  be  said  concerning  the  place  of  absorption  and  the 
minute  anatomy  of  the  structures  involved.  There  is  a 
measure  of  absorption  from  the  stomach.  Until  rather 
recently  this  organ  was  not  credited  with  any  marked 
powers  of  this  kind,  unless  it  were  in  the  case  of  alcohol. 
This  is  a  highly  diffusible  compound  and  its  prompt 
entrance  into  the  circulation  is  noteworthy.  When  one 
says  of  a  glass  of  wine,  ''This  goes  to  my  head,"  the  state- 
ment is  literally  true.  The  alcohol  strikes  through  the 
v.alls  of  the  stomach  at  once  and  is  borne  to  all  parts  of  the 
body,  including  the  brain.  On  the  other  hand,  water  is 
not  at  all  freely  absorbed  when  it  is  taken  into  an  empty 
stomach.  It  is  known  to  pass  the  pylorus  in  practically 
9 


130  NUTRITIONAL   PHYSIOLOGY 

unchanged  volume.  Some  poisons  produce  no  positive 
effects  while  in  the  stomach,  but  exert  their  action  promptly 
when  they  pass  to  the  small  intestine. 

As  regards  the  sugars  and  the  products  of  peptic  diges- 
tion, it  is  now  believed  that  there  is  a  considerable  absorp- 
tion of  these  substances  from  the  stomach.  They  disap- 
pear most  rapidly  when  they  are  present  in  high  concentra- 
tion, and  the  process  is  promoted  by  condiments  which 
increase  the  blood  flow  under  the  gastric  mucous  mem- 
brane. When  all  reasonable  allowance  is  made  for  the 
part  played  by  the  stomach  in  absorption,  the  fact  remains 
undisputed  that  the  major  part  of  the  work  is  done  below 
the  pylorus.  Furthermore,  since,  as  has  been  said,  there 
is  usually  little  valuable  material  left  to  be  recovered  by  the 
colon,  it  becomes  evident  that  the  small  intestine  is  of 
central  importance. 

A  striking  peculiarity  of  the  small  intestine  is  the  ex- 
tension of  its  internal  surface.  This  is  effected,  first,  by 
the  numerous  cross-folds  which  cut  into  its  cavity,  and, 
second,  by  the  microscopic  projections  which  stud  its 
lining.  These  are  the  villi.  An  individual  villus  may  be 
described  as  a  minute  finger-shaped  process.  It  rises 
above  the  general  surface  in  a  contrast  to  the  glands,  which 
sink  below;  the  villus  is  a  peg,  as  the  gland  is  a  pit.  Ob- 
viously the  existence  of  the  villi  increases  many  times  over 
the  number  of  cells  in  contact  with  the  intestinal  contents. 
These  cells  are  described  as  columnar;  they  are  prisms 
standing  side  by  side,  with  their  larger  surfaces  in  contact 
and  their  smaller  ends  presented  to  the  interior  of  the  in- 
testine and  to  the  loose  internal  tissue  of  the  villi.  A 
certain  share  of  absorption  may  take  place  through  the 
crevices  between  the  cells,  but  the  main  transfer  of  material 
seems  to  be  through  their  own  protoplasmic  bodies. 

The  interior  of  a  villus  is  filled  by  a  confusing  assortment 
of  cells,  some  of  w^hich  have  been  thought  to  be  contractile. 
There  are  probably  intervals  between  these  cells  containing 
lymph,  and  near  the  central  axis  of  the  villus  is  a  rather 
definite  lymphatic  channel.     It  is  a  small  branch  of  the 


THE    ABSORPTION    OF   THE    FOOD-STUFFS  131 

general  lymphatic  system  and  opens  one  way,  by  which 
food  can  pass  from  the  seat  of  absorption  to  the  veins  in 
the  thorax,  there  to  mingle  with  the  blood.  Between 
the  exposed  cells  of  the  villus  and  the  lymphatic  at  its 


Fig.  20. — This  is  a  conventionalized  drawing  to  show  the  essen- 
tials in  the  structure  of  a  villus.  The  lining  cells  of  the  intestine  are 
shown  as  in  section.  Within  is  seen  a  tangle  of  capillaries,  and  at  the 
very  core  of  the  villus  a  lymphatic  (L).  The  loose  tissue,  which  in 
reality  exists  inside  the  villus,  has  been  ignored  for  the  sake  of  sim- 
plicity. 

core  is  interposed  a  net  of  capillaries  carrying  blood  which 
has  come  from  the  neighboring  aorta,  and  which  will 
flow  through  the  liver  before  it  returns  to  the  heart.  It 
may  be  said  at  once  that,  of  the  two  possible  routes  for 
absorption,  the  portal  system  is  the  more  important. 


132  NUTRITIONAL    PHYSIOLOGY 

The  statement  has  been  made  that  the  cells  lining  the 
intestine  do  not  act  in  a  way  that  can  be  imitated  by  life- 
less models.  The  fact  is  covered  by  the  expression  that 
they  exercise  selective  powers.  Too  much  might  easily  be 
inferred  from  this  phrase;  there  is  the  same  danger  which 
was  pointed  out  in  connection  with  the  pyloric  sphincter, 
the  inclination  to  credit  the  cells  with  something  more  like 
intelligence  than  it  is  right  to  assume  for  them.  It  is 
reasonable  to  believe  that  however  complex  and  unex- 
pected may  be  the  behavior  of  the  cells  concerned,  a 
mechanistic  explanation  would  be  apparent  if  our  knowl- 
edge were  sufficiently  full.  What  is  meant  by  selective 
action  can  be  readily  illustrated. 

If  a  comparison  is  made  in  the  laboratory  to  determine 
the  relative  rates  at  which  glucose  and  magnesium  sul- 
phate make  their  way  through  an  ordinary  membrane,  the 
sugar  will  lag  behind  the  salt,  although  both  pass  with 
relative  freedom.  If  a  mixture  of  the  two  is  introduced  into 
the  living  intestine  the  impression  is  totally  different. 
The  sugar  is  absorbed,  while  the  magnesium  sulphate  is 
kept  back.  We  say  that  the  mucous  membrane  is  imper- 
meable to  this  salt,  though  we  can  hardly  picture  the 
peculiarity  of  structure  which  makes  it  so.  A  salt  which 
is  refused  absorption  by  the  intestinal  wall  will  act  as  a 
laxative,  for  it  will  hold  back  from  absorption  a  large  quan- 
tity of  water  and  this  will  be  swept  through  by  the  peris- 
talsis. An  investigator  has  called  attention  to  the  fact 
that  all  the  common  precipitants  of  calcium  are  denied 
passage  into  the  blood,  and  may,  therefore,  be  reckoned 
as  cathartics.  These  include,  beside  the  sulphates,  the 
phosphates,  the  citrates,  and  the  tartrates. 

Again  and  again  we  find  as  we  pursue  the  subject  that 
laboratory  tests  give  us  little  indication  of  what  may  be 
expected  of  the  intestine  as  an  absorbing  mechanism. 
Some  substances  usually  held  to  be  indiffusible  pass  into 
the  circulation  with  comparative  readiness.  Even  egg- 
albumen,  a  protein  of  enormous  molecule,  may  enter  the 
blood.    This  was  recently  proved  by  the  observation  that 


THE    ABSORPTION    OF   THE    FOOD-STUFFS  133 

after  eating  a  number  of  raw  eggs  the  albumin  may  be 
found  in  the  urine.  It  evidently  reaches  the  capillaries  to 
some  extent  before  it  can  be  hydrolyzed  by  the  digestive 
enzymes,  even  at  a  time  when  these  are  presumably  pres- 
ent and  active.  The  application  of  mild  poisons  to  the 
lining  of  the  intestine  causes  it  to  behave  much  more  Hke 
the  typical  indifferent  membrane.  Thus  a  weak  solution 
of  sodium  fluorid  (which  does  not  visibly  disorganize  the 
cells)  wipes  out  the  selective  properties  which  have  been 
instanced.  This  makes  it  seem  the  more  probable  that 
the  normal  processes  require  the  application  of  energy,  and 
that  accordingly  they  cannot  continue  after  the  death  of 
the  cells. 

A  mosaic  surface  formed  of  cells,  each  one  of  which  is  a 
living  body,  is  far  from  comparable  with  a  homogeneous 
partition.  Even  if  we  leave  out  of  account  the  crevices 
between  the  cells  which  may  bear  a  part  in  absorption,  as 
already  noted,  we  must  consider  the  individual  cell  to  be 
an  elaborate  structure.  Its  surface  layer  is  undoubtedly 
different  in  its  chemical  nature  from  its  interior.  There  is 
good  reason  to  believe  that  the  exposed  border  differs 
entirely  from  the  end  which  abuts  on  the  connective  tissue. 
A  result  of  this  complex  organization  is  the  existence  of  a 
property  that  might  be  called  ' 'polarity,"  that  is,  a  capacity 
to  act  in  one  direction  rather  than  the  other.  A  somewhat 
ponderous  expression  for  the  same  idea  is  found  in  the 
phrase  "irreciprocal  permeability."  This  means  special- 
ization for  absorption,  and  strongly  suggests  that  if  the 
cells  could  be  reversed  in  their  relation  to  the  interior  of  the 
intestine  they  would  begin  to  absorb  from  the  lymph  and 
secrete  into  the  canal.  Gland-cells  may  be  said  to  have 
irreciprocal  permeability  in  this  reversed  sense. 

The  departure  of  the  intestinal  lining  from  the  behavior 
of  a  common  membrane  is  still  more  obvious  when  we  note 
that  absorption  may  be  accompanied  by  chemical  trans- 
formation. The  food-stuffs  which  leave  the  interior  of  the 
canal  do  not  of  necessity  reappear  in  the  circulation  in  the 
same  form.     Two  possibilities  exist:  either  the  process  of 


134  NUTRITIONAL   PHYSIOLOGY 

digestive  cleavage  may  be  continued  during  the  passage 
through  the  wall,  or  a  recombining  of  the  products  of  diges- 
tion msiy  be  accomplished.  The  first  action  is  thought  to 
occur  to  some  extent  when  the  peptones  formed  in  the 
cleavage  of  proteins  are  moving  toward  the  blood.  It  will 
be  recalled  that  the  enzyme  erepsin,  having  the  power  to 
decompose  peptones  still  farther,  is  obtainable  from 
these  cells,  and  it  is  quite  possible  that  its  action  is  intra- 
cellular. 

The  opposite  property,  that  of  synthesizing,  is  illustrated 
by  the  behavior  of  the  products  of  fat  digestion.  We  have 
seen  that  the  pancreatic  enzyme  hydrolyzes  fats,  with  the 
formation  of  fatty  acids  and  glycerin  as  the  first  result  of 
the  cleavage,  and  that  the  fatty  acid  may  be  changed  to 
soaps,  though  we  do  not  know  how  extensively  they  under- 
go this  second  change.  It  follows  that  it  is  these  bodies 
which  disappear  from  the  intestine,  but  they  are  not  to  be 
found  at  all  freely  in  the  contents  of  the  lymphatics. 
In  their  place  we  have  neutral  fat  evidently  reconstructed 
during  the  transfer.  Microscopic  study  of  the  cells 
through  which  the  material  has  been  passing  shows  that 
the  border  adjoining  the  cavity  of  the  intestine  is  without 
fat  droplets,  but  that  these  occur  deeper  down,  increasing 
in  size  toward  the  other  border.  The  appearance  strongly 
indicates  that  some  compounds  other  than  neutral  fats 
entered  the  cells  and  underwent  a  transformation  while 
progressing  through  the  protoplasm. 


CHAPTER  XV 

THE    METABOLISM    OF    FATS    AND    CARBOHY- 
DRATES 

It  was  pointed  out  early  in  our  treatment  of  the  subject 
that  foods  serve  a  twofold  purpose:  to  some  extent  they 
are  built  into  the  body  as  relatively  permanent  parts  of  its 
structure,  while  in  a  much  larger  degree  they  are  steadily 
oxidized,  yielding  their  energy  to  maintain  its  activities. 
The  proportion  between  the  two  divisions  of  the  supply 
cannot  be  constant.  Clearly,  the  fraction  of  the  diet 
devoted  to  construction  must  be  larger  in  childhood  than 
in  adult  life.  There  may  be  other  periods  during  which  the 
constructive  work  is  notably  prominent,  for  example,  re- 
covery from  illness  or  from  fasting,  pregnancy,  and  perhaps 
athletic  training.  Apart  from  these  times  we  must  assume 
that  the  actual  building  of  tissue  is  a  very  small  item.  In 
other  words,  the  living  matter  of  the  body  is  comparatively 
stable  and  needs  only  slight  though  perfectly  definite 
contributions  to  insure  its  up-keep  from  day  to  day. 

The  food-stuffs  entering  the  circulation  may  be  destined 
for  immediate  destruction  or  for  storage.  Throughout 
long  terms  of  our  lives  a  fair  balance  is  preserved  between 
the  income  and  the  consumption  of  these  compounds. 
If  we  receive  in  one  day  certain  quantities  of  proteins, 
fats,  and  carbohydrates,  and  there  is  evidence  of  an  exactly 
equal  decomposition  of  the  three  classes  of  material,  we 
cannot  say  with  precision  whether  the  oxidation  affected 
the  particular  food  eaten  or  corresponding  matter  stored 
previously,  but  in  either  case  the  condition  of  the  system 
at  the  beginning  and  at  the  end  of  the  twenty-four  hours 
is  the  same.  We  must  now  proceed  to  discuss  the  possibil- 
ities of  transformation  and  retention  of  the  different  food- 

135 


136  NUTRITIONAL   PHYSIOLOGY 

stuffs,  and  we  shall  find  that  the  story  is  most  simple  in  the 
case  of  the  fats. 

Fat  Metabolism. — Broadly  speaking,  we  can  say  that 
fat  never  becomes  anything  else  until  it  is  decomposed  with 
release  of  energy.  This  statement  may  be  too  sweeping 
to  cover  all  the  conditions  which  may  arise  in  disease, 
but  it  is  substantially  true  in  health.  We  have  traced  the 
fat  from  the  walls  of  the  intestine  into  the  lymphatics. 
From  the  smaller  branches  it  must  find  its  way  to  the 
thoracic  duct,  and  through  this  vessel  it  goes  to  merge  with 
the  general  blood-stream.  In  the  blood,  the  lymph,  and 
the  tissues  at  large  fat  is  present  in  a  small  percentage. 
We  must  now  give  attention  to  the  special  provision  made 
for  the  storage  of  fat  in  what  is  called  adipose  tissue. 

The  word  ''fat"  is  used  in  two  senses.  In  its  strict 
chemical  meaning  it  describes  a  certain  type  of  compound, 
and  it  is  this  usage  which  we  have  thus  far  employed. 
But  when  we  speak  of  the  fat  of  meat  we  include  something 
more.  We  mean  a  form  of  connective  tissue  in  which  the 
cells  hold  a  large  accumulation  of  fat  in  the  chemical  sense. 
Under  the  microscope  this  tissue  is  seen  to  be  composed  of 
a  fibrous  network  holding  within  its  meshes  these  distended 
cells.  The  fat  which  they  contain  is  in  drops  of  such  a  size 
that  the  protoplasmic  portion  of  each  cell  seems  a  mere 
envelope,  while  the  nucleus  is  crowded  to  one  side.  So  it 
happens  that  while  the  fat  is  really  an  inclusion,  it  forms  a 
very  large  percentage  of  the  whole  mass.  When  a  piece 
of  adipose  tissue  is  subjected  to  the  action  of  gastric  juice 
the  fibers  and  protein  of  the  cells  are  rapidly  digested,  and 
the  actual  fat,  being  liberated,  rises  to  the  top  and  floats 
as  a  clear  layer  of  oil. 

Of  course,  the  amount  of  adipose  tissue  varies  widely 
with  the  individual.  Still  it  is  more  abundant  in  subjects 
of  spare  build  than  is  usually  supposed.  The  hollow  shafts 
of  the  long  bones,  such  as  those  of  the  arms  and  legs,  con- 
tain what  is  called  the  white  marrow.  This  is  typical 
adipose  tissue.  A  large  deposit  is  to  be  found  at  the  back 
of  the  abdominal  cavity,  where  it  closes  round  the  upper 


THE    METABOLISM    OF    FATS    AND    CARBOHYDRATES    137 

parts  of  the  kidneys.  Flakes  of  it  occur  in  the  mesentery 
and  on  the  surface  of  the  heart.  It  is  developed  in  the 
deep  eye-sockets.  In  subjects  better  nourished  there  will 
be  more  or  less  of  this  tissue  widely  distributed  over  the 
body  occupying  a  position  between  the  skin  and  the 
underlying  muscles — the  so-called  subcutaneous  fat. 
This  may  be  indefinitely  increased  in  the  obese.  Another 
characteristic  of  obesity  is  the  gathering  of  adipose  tissue 
in  the  great  omentum,  the  sheet  of  membrane  hanging 
from  the  lower  border  of  the  stomach.  When  much  fat 
is  present  in  this  situation  the  ventral  wall  of  the  body  has 
a  double  burden,  one  layer  of  this  reserve  material  out- 
side and  a  second  within  the  abdominal  muscles. 

We  shall  postpone  to  a  later  time  any  discussion  of  the 
factors  which  influence  the  accumulation  of  fat  in  the  sys- 
tem. A  point  previously  made  is  to  be  insisted  upon,  that 
the  fat  of  the  body  is  not  derived  solely,  nor,  indeed,  chiefly, 
from  the  fat  of  the  food.  We  shall  presently  consider  to 
what  extent  it  is  formed  from  the  other  food-stuffs. 
Whether  there  is  a  great  or  only  a  moderate  amount,  it 
win  serve  to  maintain  the  activities  of  the  muscles  during 
periods  of  insufficient  feeding  or  absolute  fasting.  When 
an  animal  has  died  from  starvation  but  little  fat  can  be 
found  in  its  tissues.  The  reduction  of  the  fat  presumably 
present  at  the  beginning  of  inanition  has  been  estimated  to 
have  reached  97  per  cent,  of  the  supply  when  death  finally 
supervenes.  The  power  to  endure  starvation  is  naturally 
greater  for  an  animal  or  a  man  having  a  large  initial  store. 

For  each  species  the  body  fat  has  a  nearly  constant 
character.  There  is  a  certain  ratio  maintained  between 
the  several  fatty  acids,  and,  as  a  result  of  this,  a  definite 
melting-point.  There  cannot  be  such  a  diversity  here,  as  is 
the  case  with  the  proteins  of  different  animals,  but  there 
is  a  similar  appearance  of  individuality.  Accordingly, 
when  one  animal  preys  upon  another  of  a  different  species 
and  is  nourished  at  the  expense  of  its  victim,  it  does  not 
store  precisely  the  form  of  fat  which  it  has  eaten,  but 
modifies  it  to  conform  to  its  own  standard.     The  making 


138  NUTRITIONAL    PHYSIOLOGY 

over  is  a  still  more  marked  phenomenon  when  a  vegetable 
oil  is  eaten  and  transformed  into  animal  fat. 

The  Metabolism  of  Carbohydrates. — The  sugar  of  the 
blood  is  usually  called  dextrose  or  glucose.  As  a  matter  of 
fact,  it  cannot  be  strictly  correct  to  speak  of  a  single  sugar 
in  the  plasma,  for  there  are  probably  three  at  least. 
Glucose,  however,  is  undoubtedly  the  principal  one,  and 
so  far  as  we  know  the  possible  services  to  the  body  are  prac- 
tically the  same  for  all.  Reference  has  already  been  made 
to  the  fact  that  the  quantity  of  sugar  in  the  blood  is  small, 
but  singularly  constant.  It  is  now  time  to  explain  how  this 
constancy  is  maintained. 

It  has  been  stated  that  the  body  contains  relatively 
little  carbohydrate  in  spite  of  its  large  supply.  When  it 
is  considered  that  the  entire  volume  of  blood  contains  less 
than  10  grams  of  sugar,  though  the  amount  absorbed  after 
a  single  meal  may  be  as  much  as  100  grams,  it  appears 
strange  that  there  should  be,  as  a  rule,  no  significant  in- 
crease in  the  percentage  circulating  during  the  period  of 
digestion.  The  solution  of  this  problem  was  achieved  in 
great  measure  by  the  French  physiologist  Bernard,  near 
the  middle  of  the  last  century.  Knowing  that  the  incom- 
ing sugar  passes  to  the  liver,  he  anticipated  that  this  organ 
might  have  the  power  to  take  the  surplus  from  the  passing 
stream  and  store  it  temporarily  in  some  form. 

Glycogen. — Investigation  showed  that  there  could  be 
obtained  from  the  liver  of  a  well-fed  animal  (rabbit)  con- 
siderable quantities  of  a  carbohydrate  resembling  starch. 
This  substance  is  called  glycogen.  Its  presence  within  the 
cells  of  the  liver  can  be  demonstrated  in  microscopic  prep- 
arations. Its  molecule  is  of  unknown  size,  and  it  is 
capable  of  undergoing  digestion  in  the  same  manner  as 
vegetable  starch  with  the  formation  of  sugar.  The  amount 
may  be  strikingly  large,  reaching,  in  the  rabbit,  one-fourth 
the  total  weight  of  the  liver,  deducting  the  weight  of  the 
blood  usually  contained  in  it.  In  the  human  liver  it  does 
not  attain  to  such  a  high  percentage,  but  may  still  equal 
something  like  10  per  cent,  of  the  net  weight  of  the  organ. 


THE    METABOLISM    OF    FATS    AND    CARBOHYDRATES    139 

This  means  that  a  full-sized  liver  may  hold  150  grams  of 
glycogen. 

Bernard's  interpretation  of  his  discovery  was  somewhat 
as  follows :  The  liver  is  the  carbohydrate  bank  of  the  body. 
Like  any  bank,  it  is  subject  by  turns  to  deposit  and  with- 
drawal. Its  hoard  is  increasing  when  much  sugar  is  ar- 
riving from  the  intestine,  for  it  is  then  diverting  the  surplus 
from  the  blood  of  the  portal  circulation.  The  change  by 
which  sugar  is  made  into  glycogen  is  clearly  just  the  re- 
verse of  the  digestive  process,  a  dehydration  and  a  con- 
densation to  form  larger  molecules  as  contrasted  with  the 
familiar  hydrolytic  cleavage.  The  liver  cells  seem  to  be 
stimulated  to  make  this  change  by  the  rise  of  the  per- 
centage of  sugar  in  the  portal  blood.  When  absorption 
ceases  it  may  be  assumed  that  the  sugar  of  the  blood  in 
general  sinks  slightly  in  amount.  This  condition  appears 
to  cause  a  reversal  of  the  prevailing  reaction  in  the  liver, 
the  stored  glycogen  is  gradually  transformed  to  sugar, 
and  this  passes  out  to  renew  the  supply  in  the  circulation. 
The  approximate  constancy  of  the  sugar  in  the  blood  is  thus 
accounted  for  in  the  main  by  the  power  which  the  liver 
possesses  to  remove  or  return  it,  according  to  the  shifting 
conditions. 

The  making  of  glycogen  from  sugars  occurs  only  during 
life.  The  converse  change  from  glycogen  to  sugar  takes 
place  freely  after  death,  and  is  doubtless  due  to  an  enzyme. 
It  might  be  anticipated  that  a  comparatively  short  period 
of  fasting  would  suffice  to  exhaust  the  glycogen  of  the  liver. 
As  a  matter  of  fact,  there  is  a  large  reduction  in  the  first 
day,  but  the  removal  then  proceeds  slowly  and  is  scarcely 
ever  completed.  The  disappearance  is  greatly  hastened  by 
muscular  activity,  most  effectually  by  the  intense  con- 
vulsions produced  by  strychnin-poisoning.  For  human 
subjects  it  has  been  shown  that  glycogen  is  consumed 
rapidly  under  the  influence  of  iced  baths.  (How  we  are 
able  to  judge  of  the  abundance  or  scarcity  of  glycogen  in 
living  men  will  be  explained  in  another  connection.) 

For   some    time  after   Bernard    first   called    attention 


140  NUTRITIONAL   PHYSIOLOGY 

to  the  ''glycogenic  function"  of  the  liver,  the  fact  that  gly- 
cogen is  deposited  also  in  the  skeletal  muscles  was  over- 
looked. In  these  tissues  it  does  not  reach  any  such  per- 
centage as  may  be  found  in  the  liver,  but  inasmuch  as  the 
muscles  form  nearly  half  the  entire  mass  of  the  body,  a 
small  percentage  means  a  large  aggregate.  Collectively, 
the  muscles  have  commonly  been  estimated  to  hold  an 
amount  equal  to  that  in  the  liver,  and  there  is  a  growing 
impression  that  they  contain  even  more.  The  total  gly- 
cogen in  the  system  may  probably  be  as  much  as  400 
grams,  or  nearly  a  pound.  The  question  which  now  calls 
for  consideration  concerns  the  importance  of  maintaining 
such  a  strict  constancy  in  the  sugar-content  of  the  blood. 
Some  light  is  thrown  on  this  matter  when  we  observe 
the  result  of  artificial  increase  of  sugar  concentration. 
This  may  be  brought  about  by  injecting  sugar  solution 
into  the  blood-vessels.  If  this  is  done  freely  there  is 
excessive  Ijmiph-formation  and  other  evidence  of  deranged 
conditions  in  the  circulatory  system.  A  symptom  to 
which  especial  attention  must  be  called  is  the  appearance 
of  sugar  in  the  urine  under  such  circumstances.  The 
kidneys  are  so  organized  that  any  distinct  rise  of  sugar  in 
the  blood  leads  to  the  excretion  of  the  excess.  Thus  the 
composition  of  the  blood  is  restored  to  the  standard,  while 
potential  food  is  lost  to  the  tissues.  Such  a  waste  of  sugar 
is  less  likely  to  follow  abundant  feeding  of  foods  rich  in  it 
than  to  occur  after  the  experimental  procedure  just  de- 
scribed, but  it  may  nevertheless  result  from  the  selection 
of  peculiar  diets.  It  is  then  called  alimentary  glycosuria. 
This  escape  of  surplus  sugar  is  not  to  be  confused  with 
diabetes.  Alimentary  glycosuria  is  induced  somewhat 
readily  by  eating  sugar,  but  hardly  ever  by  eating  starch. 
The  differing  reaction  is  presumably  explained  by  the  fact 
that  sugar  requires  a  brief  digestive  treatment  and  is  then 
rapidly  absorbed.  Starch,  on  the  other  hand,  has  to  pass 
through  serial  stages  of  digestion,  and  the  absorption  of 
the  resulting  sugar  is  extended  over  a  longer  period.  In 
the  first  case  we  may  suppose  that  the  inrush  of  sugar  over- 


THE    METABOLISM    OF    FATS    AND   CARBOHYDRATES    141 

whelms  the  Hver,  which  is  unable  to  arrest  all  of  it.  That 
which  goes  by  raises  the  sugar  content  of  the  blood  above 
the  level  at  which  it  begins  to  pass  into  the  urine. 

If  for  any  reason  much  of  the  glycogen  in  the  liver  or 
the  muscles  is  quickly  resolved  into  sugar  the  blood  must 
be  affected  quite  as  though  the  added  sugar  had  come  from 
the  intestine.  Glycosuria  will  ensue.  Certain  changes  in 
the  circulation  are  known  to  cause  such  a  flooding  of  the 
system  with  sugar  and  the  appearance  of  a  part  of  it  in  the 
urine.  A  most  interesting  instance  of  such  glycosuria  is 
that  following  an  experience  of  strong  emotion.  It 
seems  that  one  of  the  results  of  the  disturbance  in  the 
central  nervous  system  is  the  conversion  of  a  large  amount 
of  glycogen  into  dextrose.  With  acute  insight  a  physiolo- 
gist has  pointed  out  that  this  release  of  sugar  is  a  helpful 
reaction  under  the  circumstances.  The  occasion  of  emo- 
tion is  usually  an  occasion  for  strenuous  action,  perhaps  for 
flight  or  for  giving  battle,  and  the  muscles  may  be  re- 
inforced by  the  increased  supply  of  their  preferred  fuel 
brought  to  them. 

The  regulating  action  of  the  liver  and  the  muscles  upon 
the  carbohydrate  distribution  may  be  paralleled,  in  part  at 
least,  by  an  analogy.  Let  us  compare  the  active  tissues  to 
a  mill  turned  by  the  waters  of  a  stream.  The  water- 
supply  to  the  mill  is  to  be  compared  with  the  sugar-supply 
to  the  cells  which  derive  their  energy  from  it.  A  meal  is  to 
the  body  as  a  storm  is  to  the  mill-stream — it  adds  to  the 
volume  of  the  power-producing  element.  The  dam  by  the 
mill  is  like  the  kidney  in  its  relation  to  the  accumulated 
store;  if  the  water  rises  above  the  crest  of  the  dam  it  flows 
over  and  passes  on  down  the  stream  without  having  con- 
tributed its  energy  to  the  turning  of  the  machinery;  if 
the  sugar  rises  above  a  certain  level  it  begins  to  escape, 
with  its  potency  for  work  lost  to  the  organism.  More- 
over, the  capacity  of  the  liver  and  the  muscles  to  hold  back 
carbohydrate  suggests  the  function  of  a  broad  mill-pond. 
The  larger  the  pond  above  the  dam  the  more  successfully 
the  irregularities  due  to  alternating  rain  and  drought  will 


142  NUTRITIONAL    PHYSIOLOGY 

be  offset  and  the  less  likely  will  be  a  wasteful  overflow  when 
a  storm  follows  a  term  of  low  water.  The  conversion  of  an 
intermittent  supply  into  a  constant  one  is  the  function  of 
the  mill-pond,  and  it  is  equally  the  service  of  the  tissues 
holding  glycogen. 

While  a  relatively  sudden  rise  of  sugar  in  the  circulation 
may  produce  glycosuria,  a  slight  chronic  excess  over  the 
normal  may  have  an  entirely  different  effect.  If  the  diet 
is  supplying  day  by  day  a  little  more  carbohydrate  than 
the  body  is  oxidizing,  the  surplus  may  be  transformed  to 
fat.  The  opinion  that  starchy  and  saccharin  foods  are 
fattening  has  scientific  support  as  well  as  common  observa- 
tion in  its  favor.  Glycogen  formation  is  limited  and  we 
may  suppose  that  fat-building  takes  place  when  the  gly- 
cogen reserve  is  at  its  maximum  and  still  more  sugar  is 
offering.  An  important  advance  was  made  in  physiologic 
knowledge  when  Liebig  called  attention  to  the  fact  that  a 
cow's  milk  contains  an  amount  of  fat  utterly  out  of  pro- 
portion to  the  scanty  supply  furnished  by  the  food  of  the 
animal.  It  had  been  believed  that  no  such  transformation 
could  be  accomplished  by  animal  tissues,  and  that  all  fat 
found  in  the  body  or  in  the  secretions  must  have  been  re- 
ceived as  fat.  Liebig  fell  into  error  when  he  stated  that 
the  milk-fat  had  been  made  solely  from  proteins  and  not  at 
all  from  carbohydrates,  but  he  had  taken  a  notable  step  in 
recognizing  the  possibility  of  changing  one  food-stuff  into 
another.  Quantitative  experiments  soon  showed  that  car- 
bohydrates must  be  given  a  very  prominent  place  among 
fat-forming  materials. 

The  steps  through  which  sugar  is  transformed  into  fat 
are  little  understood.  A  comparison  of  the  composition 
of  the  two  makes  it  evident  that  a  great  deal  of  oxygen  has 
to  be  removed  in  the  reactions.  This  element  is  never  set 
free  in  animal  metabolism;  in  the  present  instance  it  is 
separated  in  the  form  of  carbon  dioxid.  This  is  not  merely 
a  theoretic  consideration,  but  a  condition  which  can  be 
demonstrated  by  experiments  upon  an  animal  rapidly 
gaining  fat.     A  wood-chuck,  for  example,  when  it  is  eating 


THE    METABOLISM    OF    FATS    AND    CARBOHYDRATES    143 

voraciously  of  starchy  food  and  gaining  steadily  in  fat, 
breathes  out  more  carbon  dioxid  than  can  be  accounted 
for  by  the  oxygen  consumed.  The  excess  is  a  by-product 
of  the  process  in  which  sugar  with  its  high  percentage  of 
oxygen  is  made  over  into  fat  with  a  much  lower  per- 
centage.^ 

Recourse  may  be  had  once  more  to  analogy.  Bernard 
compared  the  liver  to  a  bank,  but  we  may  extend  the  com- 
parison to  the  whole  body.  The  products  of  digestion  are 
the  daily  deposits;  the  oxidations  stand  for  the  daily 
payments.  The  bank  will  have  a  convenient  cash  balance 
on  hand  from  which  to  meet  current  demands.  This  is 
the  function  of  glycogen.  The. cash  in  the  bank  will 
be  but  a  small  fraction  of  its  total  resources,  and  its  varia- 
tions from  hour  to  hour  will  signify  but  little  as  regards  the 
stability  of  the  institution.  So  the  glycogen  of  the  body 
is  a  small  reserve  and  may  vary  by  50  per  cent,  within 
twenty-four  hours.  The  body-fat,  like  the  securities  held 
by  the  bank,  is  a  large  accumulation  and  less  subject  to 
change.  If.  for  some  time  the  deposits  are  in  excess  of  the 
withdrawals,  the  officials  of  the  bank  will,  of  course,  make 
new  investments.  The  parallel  is  clear :  the  body  receiving 
more  carbohydrate  than  it  is  expending  will  not  allow  it  to 
go  on  increasing  in  the  liver  and  muscles,  but  will  begin  to 
convert  it  to  fat.  Unhappily,  the  correspondence  fails 
at  one  point:  a  bank  which  is  subject  to  a  '^run"  may  sell 
its  bonds  and  other  holdings  that  it  may  have  cash  to 
pay  its  depositors.  The  obvious  suggestion  is  that  the 
fat  of  the  body  will  be  reconverted  to  sugar  when  there  are 
demands  to  be  met  and  no  incoming  food.  This  is  not 
known  to  occur ;  the  oxidation  of  fat  during  starvation  ap- 
pears to  proceed  without  any  such  previous  change. 

Using  another  metaphor,  though  still  a  financial  one,  it 

1  Without  assuming  that  the  process  is  fully  understood,  French 
authorities  have  suggested  that  the  principles  involved  may  be  shown 
in  the  following  equation: 

ISCCeHiA)  =  C55H,oA  +  23(C02)  +  26(H,0). 
(Shafer,  "  Text-book  of  Physiology,"  vol.  i,  p.  933.) 


144  NUTRITIONAL    PHYSIOLOGY 

may  be  said  that  the  glycogen  is  Hke  a  checking  account 
which  a  man  uses  to  pay  his  routine  expenses,  drawing 
upon  it  often  and  recruiting  it  at  longer  intervals.  Such 
an  account  is  sometimes  nearly  wiped  out  and  then  at  one 
stroke  largely  increased.  The  fat  of  the  body  is  like  a 
savings  bank  deposit,  gathered  slowly,  drawn  upon  only 
in  emergencies,  and,  it  may  be  added,  gaining  by  com- 
pound interest  in  many  cases. 

Two  facts  readily  suggest  themselves  which  may  be 
used  to  explain  the  low  limit  of  glycogen  storage.  For 
one  thing,  its  physical  properties  would  probably  make  a 
high  percentage  of  it  undesirable.  In  the  second  place, 
there  is  a  distinct  economy  in  substituting  fat  for  glycogen 
because  fat  represents  more  energy  in  proportion  to  its 
mass.  An  individual  who  carries  20  pounds  of  adipose 
tissue  may  wish  to  be  rid  of  a  part  of  it,  but  if  he  were  com- 
pelled to  bear  a  load  of  glycogen  equivalent  in  energy  his 
burden  would  amount  to  about  45  pounds. 

The  Pancreas  and  Carbohydrate  Metabolism. — We 
have  repeatedly  compared  the  carbohydrate  of  the  body 
to  money.  Just  as  it  is  the  eventual  function  of  money  to 
be  spent,  so  it  is  the  destiny  of  carbohydrate  to  be  oxidized 
that  its  latent  energy  may  be  turned  to  account.  This 
oxidation  takes  place  chiefly  in  the  muscles,  to  some  extent, 
doubtless,  in  the  glands,  the  gray  matter  of  the  nervous 
system,  and  the  absorbing  cells  of  the  intestine.  The  in- 
tensity of  the  local  process  will  in  every  case  be  proportional 
to  the  heat  and  other  forms  of  energy  developed.  Consid- 
ering the  wide  distribution  of  this  purposeful  destruction 
of  sugar  it  is  surprising  to  find  it  all  dependent  upon  an 
obscure  action  of  the  pancreas. 

This  organ  has  been  dwelt  upon  previously  as  a  most  im- 
portant contributor  of  digestive  juice  to  the  intestine.  It 
would  be  anticipated  that  the  removal  of  the  pancreas 
from  an  animal  would  be  followed  by  defects  of  digestion 
and  assimilation.  This  is  probably  justified  by  the  results 
of  a  trial,  but  the  effect  upon  digestion  is  overshadowed  by 
a  consequence  hardly  to  be  foreseen.     This  is  the  almost 


THE    METABOLISM    OF    FATS    AND    CARBOHYDRATES    145 

complete  loss  on  the  animars  part  of  the  power  to  oxidize 
dextrose.  In  other  words,  a  process  not  occurring  in  the 
pancreas  at  all,  but  in  the  tissues  at  large,  is  arrested  by  the 
removal  of  this  gland.  What  is  the  natural  explanation  of 
this  condition?  Evidently  that  something  proceeds  from 
the  pancreas  through  the  circulation  to  other  parts  of  the 
body,  without  which  the  cells  in  general  are  incapable  of 
decomposing  the  sugar. 

This  imperfectly  known  product  of  the  pancreas  is  an 
example  of  what  has  been  referred  to  as  a  hormone.  One 
is  tempted  to  think  of  it  as  an  enzyme,  but  it  cannot  be 
accurately  described  by  this  word.  If  it  were  a  typical 
enzyme  we  might  expect  it  to  cause  the  destruction  of 
sugar  in  solutions  to  which  it  has  been  added.  No  marked 
disappearance  of  sugar  occurs  when  the  experiment  is 
made.  In  view  of  this  it  is  wiser  not  to  commit  ourselves 
as  to  the  precise  character  of  the  body  in  question.  The 
fact  seems  to  be  that  neither  the  pancreatic  extract  by  itself 
nor  an  extract  of  muscle  will  particularly  promote  the 
oxidation  of  sugar,  but  that  a  combination  of  the  two  is 
necessary. 

Diabetes. — If,  owing  to  a  lack  of  the  pancreatic  hormone, 
the  ability  to  utilize  sugar  is  lost,  the  continued  absorption 
of  carbohydrate  will  raise  the  percentage  in  the  blood,  with 
the  result  which  always  ensues  under  this  condition.  The 
kidneys  will  steadily  excrete  the  surplus  sugar.  Unlike 
alimentary  glycosuria,  this  overflow  of  sugar  will  not  be 
limited  to  times  of  free  feeding  with  carbohydrates,  but 
will  attend  the  ingestion  of  the  most  moderate  amounts. 
(As  a  matter  of  fact,  sugar  will  still  be  excreted  when 
carbohydrates  are  excluded  from  the  diet  and  in  fasting. 
The  explanation  of  this  fact  must  be  deferred  to  the  next 
chapter.)  The  difficulty  of  keeping  up  nutrition  when  the 
cells  can  make  no  use  of  glucose  will  be  evident  in  a  measure 
even  at  this  point,  and  we  shall  find  additional  reasons 
later. 

Whether  human  diabetes  is  necessarily  associated  with 
some  disorder  of  the  pancreas  cannot  be  stated  with  cer- 

10 


146  NUTRITIONAL    PHYSIOLOGY 

tainty.  It  is  known  to  be  so  in  an  increasing  number  of  the 
cases  studied.  Often  when  the  pancreas  shows  no  defect 
to  the  naked  eye,  some  abnormahty  is  revealed  by  the 
microscope.  Assuming  that  the  trouble  centers  in  the  lack 
of  the  hormone,  physicians  have  frequently  undertaken 
to  treat  diabetes  by  administering  preparations  of  pancre- 
atic tissue  or  extracts.  The  results  have  been  generally 
disappointing,  though  a  solitary  case  recently  reported 
showed  marked  improvement  for  a  time.  The  peculiarity 
of  the  subject  in  this  instance  was  an  uncommon  capacity 
for  eating  nearly  raw  meat.  This  made  it  possible  for  him 
to  be  fed  incredible  quantities  of  pancreatic  tissue  (sweet- 
breads) from  calves.  The  helpful  treatment  was  inter- 
rupted when,  after  an  attack  of  indigestion,  he  found  him- 
self unable  to  eat  any  more  sweetbreads.  To  say  that 
he  could  not  do  so  to  save  his  life  is  to  express  the  literal 
truth,  for  he  fell  into  a  rapid  decline  and  soon  died. 


CHAPTER  XVI 
NITROGENOUS   METABOLISM 

Our  knowledge  of  the  history  of  the  products  of  protein 
digestion  has  been  much  extended  in  the  past  few  years,  but 
there  are  still  many  uncertain  passages.  Any  account 
which  can  be  given  must  be  held  subject  to  revision. 
Nevertheless  the  course  of  nitrogenous  metabolism,  in  its 
broader  aspects,  is  tolerably  clear.  The  present  interpre- 
tation is  most  easily  appreciated  when  earlier  conceptions 
have  been  briefly  reviewed. 

Not  long  ago  it  was  held  that  the  earlier  products  of 
tryptic  digestion  were  promptly  absorbed,  and  that  the 
formation  of  the  late  products,  the  amino-acids,  was  rather 
an  accidental  and,  possibly,  an  unfortunate  occurrence. 
It  was  believed  that  the  simplest  bodies  could  not  serve  all 
the  purposes  of  nutrition.  Such  compounds  were  known 
to  arise  in  the  laboratory  experiments,  but  their  formation 
in  the  intestine  under  strictly  normal  conditions  was  ques- 
tioned. The  opinion  prevailed  also  that  the  peptones 
which  disappeared  from  the  canal  were  reconstructed  at 
the  time  of  absorption  and  represented  thereafter  by  the 
protein  of  the  blood.  The  old  impression  that  but  little 
in  the  line  of  synthesis  could  be  expected  of  the  animal 
tissues  was  distinctly  influential. 

About  1901  it  was  shown  that  a  dog  can  be  nourished 
when  the  nitrogenous  food  which  it  receives  is  in  the  form 
of  the  most  advanced  products  of  tryptic  hydrolysis.  A 
quantity  of  lean  meat  had  been  digesting  with  pancreatic 
juice  for  a  period  of  months.  The  resulting  mixture 
contained  only  bodies  of  a  simpler  order  than  the  ones 
on  which  nutrition  had  hitherto  been  supposed  to  depend. 
Food  so  prepared  is  not  attractive,  but  it  can  maintain  an 

147 


148  NUTRITIONAL   PHYSIOLOGY 

animal  in  fair  condition.  Physiologists  accepted  the 
evidence  and  granted  that  the  proteins  of  the  organism 
could  be  synthesized — sometimes  at  least — from  amino- 
acids.  Shortly  after  this  change  in  our  conceptions  the 
discovery  of  erepsin  was  announced.  It  was  recognized 
that  the  existence  of  this  enzyme  made  it  more  probable 
that  digestion  should  normally  run  its  full  course  rather 
than  it  should  be  terminated  at  an  early  stage  by  the  in- 
tervention of  the  absorbing  cells.  It  became  apparent 
that  even  though  the  material  leaving  the  intestine  might 
have  the  relatively  complex  character  which  we  associate 
with  the  peptone  stage  of  digestion,  the  products  delivered 
to  the  interior  of  the  villi  by  the  cells  might  have  under- 
gone further  cleavage.  The  ability  of  the  animal  to  turn 
to  account  such  simple  bodies  in  sjnithesizing  its  own 
proteins  became  clear. 

In  the  chapter  on  Intestinal  Digestion  (Chapter  X)  the 
statement  was  made  that  the  various  amino-acids  have 
been  called  the  "building-stones"  of  metabolism.  In  the 
course  of  digestion  they  are  separated,  and  after  absorption 
or  during  the  very  act  of  absorption,  they  are  to  some 
extent  assembled  again.  Many  facts  bearing  upon  this 
process  have  been  brought  to  our  attention  by  the  recent 
studies  of  physiologic  chemists  concerning  the  constitu- 
tion of  protein  molecules.  All  that  has  been  done  of  late 
in  this  direction  has  served  to  emphasize  the  variety  of  in- 
dividual structure  comprehended  under  the  term  protein. 
When  such  compounds  from  various  sources  are  subjected 
to  decomposition,  either  by  digestion  or  by  other  means, 
the  assortment  of  amino-acids  obtained  differs  with  the 
particular  protein  under  investigation. 

Some,  which  one  is  tempted  to  regard  as  perfect  proteins, 
yield  the  full  list  of  amino-acids  as  at  present  known. 
Others,  seemingly  defective  when  judged  by  this  rather 
arbitrary  standard,  fail  to  yield  certain  members  of  the 
series.  This  variation  has  an  important  bearing  on  mat- 
ters of  nutrition.  It  can  no  longer  be  maintained  that  all 
substances  characterized  as  proteins  are  equivalent  in 


NITROGENOUS    METABOLISM  149 

their  power  to  minister  to  growth  and  repair.  Among 
an  increasing  number  of  defective  proteins  now  recognized 
gelatin  is  the  best  known.  Its  behavior  in  the  system  calls 
for  exposition. 

Gelatin. — This  familiar  compound  belongs  to  the  ill- 
defined  group  often  spoken  of  as  the  albuminoids.  These 
may  fairly  be  regarded  as  proteins  which  have  undergone 
a  more  or  less  definite  degeneration  both  in  function  and 
in  chemical  structure..  They  are  found  as  intercellular 
substance  in  the  connective  tissues  and  also  in  the  dead 
and  dry  surface  layer  of  the  skin.  They  make  the  chief 
substance  of  the  hair  and  nails.  Gelatin  itself  is  derived 
by  boiling  certain  varieties  of  connective  tissue,  including 
bone  and  tendon.  It  gives  nearly  average  percentages  of 
the  five  elements  present  in  ordinary  proteins.  When  this 
fact  was  discovered  in  the  early  days  of  organic  chemistry 
it  was  urged  that  gelatin  must  have  a  value  in  nutrition 
quite  equal  to  that  of  any  nitrogenous  food. 

Trials  were  made  on  a  large  scale  in  the  pauper  institu- 
tions of  France.  It  was  found  very  definitely  that  the 
free  use  of  gelatin  led  to  indigestion  and  that  it  soon 
became  repugnant  to  the  subjects,  however  hungry  they 
might  be.  These  effects  were  later  found  to  be  correlated 
with  inadequacy  to  maintain  the  weight  and  strength  of 
animals.  An  animal  cannot  be  said  to  be  perfectly  nour- 
ished if  it  is  losing  more  of  any  element  day  by  day  than  it 
is  receiving.  This  is  eminently  true  of  the  nitrogen  in  the 
income  and  the  outgo.  "Nitrogenous  equilibrium," 
expressing  the  equality  of  the  two,  is  a  phrase  we  shall  use 
freely.  Now  nitrogenous  equilibrium  cannot  be  estab- 
Hshed  by  feeding  gelatin  in  place  of  all  other  protein,  no 
matter  how  skilfully  the  experiment  is  conducted. 

For  many  years  the  insufficiency  of  gelatin  to  make  good 
the  losses  from  the  tissues  remained  a  mystery.  It  has 
been  amply  explained  by  the  findings  of  chemists  in  our 
own  time.  Gelatin  yields  most  of  the  building-stones  re- 
quired for  the  new  construction,  but  it  does  not  furnish 
them  all.     Therefore  it  is  impossible  to  make  blood-pro- 


150  NUTRITIONAL   PHYSIOLOGY 

teins  or  the  proteins  of  the  living  cells  from  its  cleavage 
products.  It  is  useless  to  increase  the  quantity  of  the 
amino-acids  if  the  variety  is  not  great  enough  to  supply  the 
details  of  the  molecular  pattern  to  be  wrought.  What  is 
true  of  gelatin  is  true  of  a  number  of  proteins  from  vege- 
table sources.  They  do  not  give  all  the  groupings  needed 
in  the  constitution  of  the  more  elaborate  animal  proto- 
plasm. 

In  some  cases  a  single  protein  may  be  adequate  for  nutri- 
tion, supplying  a  complete  assortment  of  amino-acids. 
But  it  will  be  seen  that  successful  nutrition  is  more  cer- 
tainly to  be  secured  by  using  proteins  from  various  foods. 
This  is  our  practice,  save  in  the  important  case  of  the  milk- 
fed  infant.  Milk  actually  contains  proteins  of  more  than 
one  order,  so  that  the  exclusive  use  of  this  food  does  not 
narrow  the  selection  of  building  units  so  greatly  as  might 
be  supposed.  The  chief  protein  of  milk  contains  the  ele- 
ment phosphorus  and  is  perhaps  of  somewhat  unusual 
complexity. 

In  an  introductory  chapter  the  protein  molecule  was 
likened  to  a  watch  with  its  many  dissimilar  parts  associ- 
ated in  the  one  possible  way  to  secure  a  desired  result. 
One  or  more  of  these  parts  might  be  missing  without  their 
absence  being  apparent  to  the  untrained  person  as  he 
looked  into  the  works.  He  could  nevertheless  observe  the 
fact  that  the  watch  would  not  go.  This  is  quite  parallel 
with  our  progress  toward  an  understanding  of  the  failure 
of  gelatin  and  other  proteins  to  serve  all  purposes  in  nutri- 
tion. Just  as  the  watch-maker,  with  his  special  knowledge, 
easily  detects  what  is  wanting,  so  the  physiologic  chemist 
is  now  able  to  say  with  much  accuracy  what  particular 
amino-acids  are  lacking  in  his  feeding  experiments.  As 
the  defective  watch  may  be  made  serviceable  by  the  addi- 
tion of  certain  bits  of  mechanism,  so  in  a  measure  an  in- 
sufficient diet  may  be  made  adequate  when  extra  amino- 
acids  are  supplied. 

It  is  necessary  now  to  approach  a  subject  of  some 
difficulty.     We  must  attempt  to  show  why  a  given  quan- 


NITROGENOUS    METABOLISM  151 

tity  of  protein  fed — say  100  grams — cannot  contribute  an 
equal  quantity  to  the  protein  supply  of  the  body.  When 
protein  of  one  kind  undergoes  complete  hydrolysis  and 
protein  of  a  new  kind  is  to  be  made  from  the  resulting 
cleavage  products,  certain  of  the  building-stones  will  be 
needlessly  plentiful,  while  others  will  be  relatively  scarce. 
We  have  seen  that  if  a  single  one  of  these  structural  units 
is  not  furnished,  there  is  complete  failure  to  synthesize 
the  new  compound.  Similarly,  if  the  second  body  is  to 
contain  a  large  percentage  of  an  amino-acid  which  is  but 
scantily  represented  in  the  first,  the  possible  formation  is 
definitely  limited.  The  principle  is  easily  illustrated. 
Suppose  that  in  a  club  of  100  members  there  are  25  Demo- 
crats. It  is  desired  to  elect  for  purposes  of  debate  the 
largest  possible  body,  consisting  of  Democrats  and  Re- 
publicans in  equal  numbers.  Evidently,  this  body  will 
comprise  25  men  of  each  party.  There  will  be  50  men  un- 
related to  the  new  organization.  We  may  change  the 
comparison:  A  house  is  pulled  do'^Ti  and  another  is  to  be 
erected  from  the  timbers.  If  the  second  house  is  of  an 
architecture  entirely  unlike  that  of  the  first,  there  will  be 
many  unavailable  pieces  to  discard  and  the  new  building 
will  be  smaller  than  the  old.  It  is  not  at  all  unlikely  that 
the  misfit  fragments  of  building  material  will  go  into  the 
cellar  of  the  new  house,  later  to  be  used  as  fuel.  This  is 
just  what  the  body  does  with  the  misfit  amino-acids.  So 
far  as  they  do  not  find  place  in  the  mosaic  which  is  put 
together  they  serve  as  producers  of  energy. 

Again,  a  better  analogy  suggests  itself :  The  structure  of 
a  molecule  of  food-protein,  previously  compared  with  that 
of  a  watch  or  a  house,  may  be  likened  to  the  type  set  up  to 
print  a  page.  The  letters,  some  of  which  occur  frequently 
and  some  rarely,  stand  for  the  amino-acids.  The  type  is 
allowed  to  fall  apart,  the  symbol  of  digestion.  It  is  to  be 
set  up  again  to  print  different  matter.  If  the  language  and 
vocabulary  are  much  as  at  first,  it  may  be  possible  to  com- 
pose nearly  a  whole  page  before  the  lack  of  some  letter 
brings  the  proceeding  to  a  standstill.     But  some  shrinkage 


152  NUTRITIONAL   PHYSIOLOGY 

will  be  inevitable  and  when  the  type-setting  has  to  halt 
there  will  be  some  unused  letters.  The  shrinkage  will  be 
much  greater  if  the  second  page  is  to  be  printed  in  a  lan- 
guage other  than  the  original.  Suppose,  for  example,  the 
type  used  in  English  composition  is  next  devoted  to  Ger- 
man. The  resulting  difficulty  is  readily  foreseen — the 
letter  z  is  uncommon  in  English,  but  frequent  in  German. 
Hardly  a  line  can  be  perfectly  set  up  before  this  letter  will 
be  vainly  sought.  Almost  the  whole  collection  of  type  will 
be  useless  for  composing.  This  is  analogous  to  the  attempt 
to  minister  to  animal  growth  with  some  isolated  vegetable 
protein  of  exceptional  constitution.  Offering  the  cleavage 
products  of  gelatin  to  the  cells  is  like  giving  the  compositor 
incomplete  fonts  of  type.  He  cannot  set  up  connected 
matter  if  some  of  the  letters  are  not  to  be  found. 

It  seems  natural  to  assume  that  the  closer  the  structural 
resemblance  between  the  proteins  digested  and  those  to  be 
synthesized,  the  more  economically  the  making  over  can 
be  accomplished.  It  is  permissible  to  infer  that  nutri- 
tion can  be  subserved  by  a  smaller  quantity  of  proteins 
when  they  are  derived  from  animal  sources  than  when 
they  are  of  vegetable  origin.  One  cannot,  however,  use 
this  as  a  strong  argument  against  vegetarianism.  The 
quantity  of  proteins  which  one  takes  when  following  the 
dictates  of  the  appetite  is  apparently  so  liberal  that  all 
constructive  requirements  are  easily  met,  even  though  the 
difference  between  the  composition  of  the  food-proteins 
and  those  of  the  body  is  a  wide  one.  Too  rigorous  logic 
applied  in  this  connection  might  lead  to  the  recommending 
of  cannibalism. 

Our  knowledge  of  the  place  and  the  manner  of  protein 
synthesis  is  incomplete.  The  cells  which  line  the  intestine 
and  receive  the  digestive  products  are  generally  held  to 
bear  a  large  part  in  the  work.  The  fact  that  these  cells 
contain  erepsin,  an  enzyme  capable  of  breaking  down  the 
more  complex  nitrogenous  bodies,  does  not  exclude  the 
possibility  that  dehydrations  and  condensations  may  still 
take  place  within  them.     The  enzyme  may  be  modified  un- 


NITROGENOUS    METABOLISM  153 

der  some  conditions  so  as  to  be  inactive.  It  is  even  con- 
ceivable that  it  may  facilitate  the  combining  of  the  build- 
ing-stones. Some  enz\^nes,  like  other  catalysts,  may  favor 
the  progress  of  reactions  either  in  one  direction  or  the 
reverse,  according  to  the  proportions  of  the  substances 
present  at  the  moment. 

It  is  not  easy  to  estimate  the  extent  of  protein  synthesis 
which  normally  takes  place.  Of  course,  it  is  more  promi- 
nent during  growth  than  during  adult  life.  The  present 
impression  is  to  the  effect  that  only  a  very  small  fraction 
of  the  usual  protein  income  is  thus  used.  ^Most  people 
can  materially  reduce  the  quantity  of  protein  in  the  diet 
and  still  remain  in  nitrogenous  equilibrium.  When  the 
lowest  level  at  which  this  is  possible  has  been  attained  it 
is  still  true,  as  we  have  just  pointed  out,  that  the  amount 
of  new  protein  constructed  is  but  a  fraction  of  that  sup- 
plied. It  is,  therefore,  certain  that  under  average  con- 
ditions by  far  the  larger  part  of  the  nitrogenous  food  eaten 
never  exists  in  the  form  of  proteins  after  absorption.  We 
must  now  consider  the  destiny  and  value  of  the  micombined 
building-stones. 

The  surplus  amino-acids,  either  free  or  in  simple  com- 
binations, are  borne  away  from  the  intestine  in  the  portal 
blood.  Accordingly,  these  digestive  products,  like  the 
sugars,  are  brought  under  the  influence  of  the  liver-cells. 
They  undergo  a  transformation  in  this  organ — and  very 
probably  elsewhere — which  has  important  consequences. 
This  is  the  process  described  as  ''deaminization."  To 
deaminize  an  amino-acid  is  to  remove  from  it  the  group 
to  which  it  owes  its  name,  the  radicle  NHo.  One  of  the 
products  of  this  reaction  will  be  non-nitrogenous,  the  other 
will  contain  nitrogen  in  a  greatly  increased  percentage. 
We  cannot  claim  to  know  all  the  steps  which  are  gone 
through  in  this  comiection,  and  we  shall  not  discuss  those 
which  are  known.  We  shall  emphasize  simply  the  final  re- 
sults. 

The  chief  nitrogenous  compound  which  issues  from  the 
series  of  reactions  occurring  in  the  liver  is  urea.     This  is 


154  NUTRITIONAL   PHYSIOLOGY 

apparently  of  no  further  use  in  the  system.  It  is  destined 
to  circulate  until  it  shall  find  its  way  into  the  urine.  The 
efficiency  of  the  kidneys  is  so  remarkable  and  the  whole 
blood  volume  is  carried  through  them  at  such  short  in- 
tervals that  the  percentage  of  urea  in  normal  blood  is 
kept  very  low.  Urea  seems  to  be  a  most  convenient  form 
for  nitrogen  elimination;  it  is  highly  soluble  and  diffusible, 
inert,  and,  comparatively  speaking,  non-poisonous.  If 
a  man  eats  100  grams  of  protein  in  twenty-four  hours  he 
will  excrete  some  30  grams  of  urea,  an  amount  which  rep- 
resents about  seven-eighths  of  the  total  nitrogen  passing 
into  and  out  of  the  body.  This  urea  is  not  all  made  by  the 
liver,  but  a  large  share  of  it  proceeds  from  the  deaminizing 
activities  of  this  organ. 

What,  then,  is  the  principal  non-nitrogenous  compound 
produced  from  the  amino-acids  in  the  liver?  There  seems 
to  be  no  doubt  that  it  is  dextrose.  Evidence  in  support  of 
this  belief  has  been  derived  from  the  study  of  diabetes. 
Whether  this  condition  is  experimentally  induced  or  devel- 
ops spontaneously,  it  is  found  that,  if  the  case  is  one  of  full 
severity,  sugar  excretion  goes  on  even  when  no  carbohy- 
drate is  fed,  and,  indeed,  throughout  long  periods  of  fast- 
ing. This  sugar  might  be  assumed  to  have  come  from  the 
fat  of  the  body,  but  it  can  be  more  surely  attributed  to  the 
protein  which  is  being  decomposed.  The  following  con- 
sideration shows  this:  the  nitrogen  and  the  sugar  in  the 
urine  of  the  fasting  diabetic  patient  maintain  a  singularly 
constant  ratio;  1  gram  of  nitrogen  is  accompanied  by  3.6 
grams  of  dextrose.  They  rise  and  fall  together.  This 
seems  to  prove  that  the  two  must  have  a  common  source, 
which  can  only  be  protein.  Again,  the  feeding  of  amino- 
acids  to  the  diabetic  increases  his  loss  of  sugar;  the  feeding 
of  fat  does  not  have  this  effect.  A  simple  calculation 
shows  that  100  grams  of  protein  fed  may  give  rise  within 
the  body  to  about  57  grams  of  sugar.  The  seriousness  of 
diabetes  will  now  be  better  appreciated  than  has  been 
possible  up  to  this  time.  The  organism  loses  not  merely 
the  support  normally  secured  from  the  chief  carbohydrates 


NITROGENOUS   METABOLISM  155 

of  the  diet,  but,  in  great  part,  that  ordinarily  furnished  by 
protein  food. 

From  all  this  it  appears  that  much  of  the  protein  which 
we  eat  serves  only  to  supply  the  tissues  with  carbohydrate. 
The  impression  is  likely  to  be  that  this  is  a  roundabout 
and  not  an  economical  way  to  provide  sugar,  which  might 
have  been  taken  as  such  at  the  outset.  The  facts  may 
fairly  be  employed  to  support  the  modern  teaching  that 
excess  of  protein  is  to  be  avoided,  but  we  have  already 
shoTNTi  the  necessity  for  allowing  more  than  is  actually  to 
be  reconstructed  after  the  digestive  dismembering.  Recog- 
nizing as  inevitable  the  discarding  of  amino-acids,  we  can 
see  the  desirability  of  having  them  made  to  furnish  a 
simple  standard  food  like  sugar,  valuable  for  its  store  of 
energy.  The  possibility  of  glycogen  formation  from  pro- 
tein naturally  follows.  The  glycogen  of  carnivorous 
animals  presumably  has  such  an  origin.  The  maintenance 
of  the  sugar  of  the  blood  during  long  fasting  is  also  ascribed 
to  the  disintegrating  protein  of  the  tissues.  Given  dex- 
trose in  such  quantities,  the  production  of  fat  from  protein 
becomes  at  least  theoretically  possible.  Broadly  speaking, 
we  may  claim  for  protein  that  it  can  do  all  that  any  form 
of  organic  food  can  do  for  the  system.  Yet  this  does  not 
impair  the  statement,  equally  to  be  recognized,  that  car- 
bohydrates and  fats  should  form  much  the  larger  part  of 
the  income. 

After  the  constructive  requirement  has  been  met,  all  ad- 
ditional protein  seems  to  entail  unprofitable  labor  on  the 
part  of  the  liver  in  deaminizing  the  cleavage  products,  the 
presence  of  various  substances  of  a  possibly  detrimental 
nature  in  the  circulation,  and  an  activity  on  the  part  of  the 
kidneys  which  may  amount  to  an  abuse  of  these  important 
excretory  organs.  There  is  this  general  contrast  between 
the  behavior  of  proteins  and  non-proteins  in  the  body: 
the  former  give  rise  to  rather  complex  waste-products  im- 
posing a  task  upon  the  liver  and  kidneys;  the  latter  are 
oxidized  cleanly  to  carbon  dioxid  and  water,  two  com- 
pounds which  are  eliminated  with  ease.     A  fuller  discus- 


156  NUTRITIONAL   PHYSIOLOGY 

sion  of  these  facts  will  be  undertaken  in  the  chapter  on  The 
Hygiene  of  Nutrition  (Chapter  XXII). 

Folin,  of  the  Harvard  Medical  School,  has  classified  the 
facts  of  protein  metabolism  in  a  particularly  clear  and 
helpful  form.  He  distinguishes  two  lines  of  transforma- 
tion, the  endogenous  and  the  exogenous.  Under  the  head 
of  endogenous  metabolism  he  traces  the  various  steps  in 
the  history  of  those  building-stones  which  are  erected  into 
the  proteins  of  the  blood  and  other  tissues.  The  narrative 
is  continued  in  the  account  of  the  rather  gradual  and  steady 
crumbling  which  such  tissue-proteins  undergo.  What  is 
loosely  called  the  wear  and  tear  of  the  cells  gives  rise  to 
definite  end-products,  one  of  which  Folin  holds  to  be  of 
particular  value  in  estimating  the  extent  of  such  decompo- 
sition. This  is  the  substance  creatinin,  which  accompanies 
urea  out  of  the  body  and  which  is  far  less  subject  to  fluctua- 
tion. The  urea  excreted  rises  and  falls  with  the  protein 
ration,  but  the  creatinin  is  not  markedly  responsive  to 
dietetic  variations.  It  is  believed,  therefore,  that  endog- 
enous metabolism,  a  necessary  feature  of  animal  life,  is 
relatively  independent  of  feeding,  at  least  while  nutrition 
is  satisfactory. 

Exogenous  metabolism  is  an  expression  to  cover  all  the 
reactions  affecting  the  uncombined  amino-acids.  Hence 
it  includes  the  formation  of  urea,  dextrose,  and  whatever 
substances  may  be  formed  from  the  nitrogenous  cleavage 
products  in  the  liver.  It  may  be  extended  to  take  in  the 
secondary  production  of  glycogen  or  of  fat  from  surplus 
sugar  originating  in  this  way.  In  contrast  with  endogen- 
ous metabolism  its  amount  varies  widely.  With  a  low 
protein  diet  the  exogenous  changes  will  be  but  a  fraction 
of  what  they  will  become  with  abundant  protein.  One 
may  be  tempted  to  conclude  that  in  fasting  the  metabolism 
will  be  wholly  endogenous.  The  insight  of  a  German  writer 
has  served  to  show  us  that  this  is  not  so.  We  have  spoken 
at  length  of  the  assembling  of  amino-acids  fresh  from  the 
intestine  to  form  the  standard  proteins  of  the  blood.  Now 
there  are  differences  of  constitution  between  the  blood- 


NITROGENOUS   METABOLISM  157 

proteins  and  those  of  the  various  organs  much  as  there  are 
between  these  same  blood-proteins  and  those  of  the  food. 
When  a  particular  tissue,  muscle,  for  example,  is  to  be 
nourished  at  the  expense  of  the  blood  (or  the  lymph),  a 
true  local  digestion  is  necessary,  and  once  more  there  must 
be  the  selecting  and  rejecting  of  amino-acids.  Those  not 
available  may  reach  the  liver  and  be  deaminized  there 
quite  as  if  they  had  come  from  the  seat  of  the  original 
digestion. 

Note. — The  outlines  of  nitrogenous  metabolism  given 
in  this  chapter  follow  closely  Abderhalden's  presentation 
of  the  subject.  It  is  impossible  to  predict  how  far  the 
prevailing  views  may  be  modified  as  the  result  of  work  now 
in  progress.  Mendel's  researches  seem  tending  to  support 
the  belief  that  more  remodeling  of  the  amino-acids  can  be 
carried  on  than  has  been  generally  supposed.  A  recent 
suggestion  of  his  must  be  considered.  The  food  which  lies 
in  the  intestine  is  not  used  solely  by  the  animal,  but  serves 
also  to  nourish  swarms  of  bacteria.  These  are  plants  and 
are  known  to  possess  well-marked  synthetic  powers.  It  is 
certain  that  the  multiplying  bacteria  construct  new  pro- 
teins from  the  nitrogenous  cleavage  products  placed  at 
their  disposal.  The  proteins  thus  formed  may  be  radically 
different  in  molecular  pattern  from  any  in  the  diet.  Great 
numbers  of  these  intestinal  bacteria  are  always  perishing, 
and  when  they  die  they  must  yield  their  substance  to  the 
digestive  juices  for  resolution  into  its  component  groupings. 
Thus  there  may  be  a  hidden  supply  of  amino-acids  to  the 
system  which  is  more  or  less  independent  of  the  ration 
w^hich  the  experimenter  has  furnished. 

Another  possible  divergence  from  the  account  given 
above  must  be  indicated.  As  we  have  pictured  the  suc- 
cession of  events,  the  amino-acids  used  for  synthesis  have 
been  combined  to  form  the  characteristic  and  abundant 
proteins  of  the  blood,  these  to  be  locally  digested  and  made 
to  supply  the  necessary  building  units  to  the  various 
tissues.    But  the  impression  seems  to  be  gaining  in  favor 


'  158  NUTRITIONAL    PHYSIOLOGY 

that  amino-acids  uncombined  may  escape  the  influence  of 
the  hver,  and  be  taken  to  all  parts  of  the  body  and  offered 
in  free  condition  to  organs  which  may  require  them.  If 
this  is  the  usual  procedure  the  suggestion  is  that  the  blood 
proteins  form  a  rather  stable  reserve  mass  of  food  material 
not  much  subject  to  depletion  and  renewal  under  ordinary 
conditions  of  feeding.  In  starvation  we  know  that  the  pro- 
teins of  certain  organs  are  used  to  keep  up  the  nutrition  of 
others  more  essential  to  survival.  It  would  be  interesting 
to  know  whether  such  transferred  material  is  carried  in  the 
form  of  amino-acids  or  organized  temporarily  into  proteins 
of  the  type  found  in  the  plasma.  But  there  is  no  part  of 
physiologic  chemistry  where  our  knowledge  is  so  much  at 
fault  as  just  this  section. 

A  SUMMARY  OF  METABOLISM 

Fats  are  hydrolyzed  in  the  alimentary  canal  to  fatty 
acids  and  glycerin.  To  an  uncertain  extent  there  is  soap 
formation.  These  products  are  largely  recombined  to 
form  fat  during  the  passage  through  the  lining  cells  of  the 
intestine.  Fat  is  stored  chiefly  in  adipose  tissue.  Its 
eventual  service  is  to  be  oxidized  to  carbon  dioxid  and 
water  with  release  of  its  energy. 

Carbohydrates  enter  the  circulation  in  the  form  of  simple 
sugars,  mainly  glucose.  There  is  little  sugar  in  the  blood 
at  any  one  time.  Much  is  dehydrated  by  the  cells  of  the 
liver  and  muscles  to  make  glycogen,  subject  to  reconversion 
to  sugar  when  required.  A  surplus  may  be  converted  to 
fat.  The  possibility  of  the  converse  change  from  fat  to 
sugar  is  generally  held  to  be  unproved.  The  final  value 
of  carbohydrate  is  like  that  of  fat;  its  energy  is  set  free 
through  the  respiratory  oxidation,  and  the  end-products 
again  are  carbon  dioxid  and  water.  The  internal  secretion 
of  the  pancreas  is  necessary  to  bring  about  this  destruction. 

Proteins  are  hydrolyzed  to  simple  compounds  (amino- 
acids  or  combinations  of  these),  and  these  are  used  for  the 
synthesis  of  the  new  proteins — of  types  peculiar  to  the 
species — which  can  be  utilized  for  growth  or  repair.     A 


NITROGENOUS    METABOLISM  159 

large  surplus  of  uncombined  amino-acids  is  usual ;  these  are 
dealt  with  by  the  liver,  and  the  best-known  resulting  pro- 
ducts are  urea — destined  to  be  excreted — and  glucose, 
for  which  all  the  possibilities  exist  that  have  been  men- 
tioned above.  The  amount  of  wasting  suffered  by  nitrog- 
enous tissues  as  a  part  of  their  life  process  is  believed  to 
to  be  indicated  by  the  extent  to  which  the  product  creatinin 
is  eliminated.  All  proteins  yield  sulphur  compounds 
among  their  decomposition  products.  Some  yield  phos- 
phorus compounds  also,  and  it  is  these  proteins  which 
give  rise  to  much  of  the  uric  acid  which  often  tends  to 
be  retained  and  to  cause  trouble. 


CHAPTER  XVII 

THE  REMOVAL  OF  THE  END-PRODUCTS  OF 
METABOLISM 

The  statement  has  repeatedly  been  made  in  varying 
form  that  the  bulk  of  the  food  is  taken  for  the  sake  of  its 
potential  energy.  Either  at  once  or  after  storage  it  is 
oxidized,  and  the  energy  turned  to  account  for  tempera- 
ture maintenance  and  for  the  performance  of  muscular 
work.  The  main  products  are  carbon  dioxid  and  water. 
These  are  likewise  the  chief  products  formed  when  familiar 
fuels  are  burned  outside  the  body.  Wood,  coal,  and  gas 
yield  the  two  in  great  quantity  and  only  small  amounts  of 
other  compounds.  Hence  the  primary  problems  of  excre- 
tion concern  the  manner  of  elimination  of  carbon  dioxid 
and  water. 

The  water  leaving  the  body  during  twenty-four  hours 
may  be  2  or  3  kilograms.  The  carbon  dioxid  discharged 
in  the  same  period  is  not  often  in  excess  of  1  kilogram. 
Nevertheless,  we  say  that  carbon  dioxid  is  the  leading 
waste-product  of  animal  life.  This  is  justified  by  the  con- 
sideration that  by  far  the  larger  part  of  the  water  which  we 
measure  is  merely  water  previously  received  in  the  same 
state.  To  this  large  volume  the  tissues  have  added  a  mod- 
erate quantity  of  water — say  250  grams — which  is  a  true 
metabolic  product.  This  has  been  formed  by  the  oxidation 
of  compounds  containing  hydrogen.  The  water  output 
of  the  body  is  inevitably  greater  in  the  long  run  than  the 
water  income.  This  fact  may  be  disguised  on  single  days 
by  water  retention. 

Carbon  Dioxid  Elimination. — Respiration  has  been 
defined  as  the  process  within  the  living  cells  in  course  of 
which  complex  organic  molecules  are  decomposed  and 

160 


REMOVAL  OF  THE  END-PRODUCTS  OF  METABOLISM   161 

more  or  less  completely  oxidized.  Carbon  dioxid  is  the 
most  conspicuous  product.  The  respiratory  exchanges 
occur  in  the  different  tissues  in  a  measure  corresponding 
with  the  extent  to  which  they  severally  evolve  energy. 
The  skeletal  muscles  lead  in  amount  of  respiration  (and 
of  carbon  dioxid  set  free),  both  because  of  their  great  mass 
and  their  activity.  The  glands,  especially  the  liver  and 
the  kidneys,  contribute  largely  to  the  total.     So  does  the 


1         Al      _ 


Cap      J^l'^°PCorp. 


Fig.  21. — I  is  intended  to  suggest  the  form  of  an  air-sac  of  the 
lung  overlaid  with  a  network  of  capillaries  belonging  to  the  pul- 
monary system.  II  is  an  imaginary  section  through  such  an  air- 
sac.  B  in  both  I  and  II  is  the  minute  bronchial  tube  through  which 
the  air  is  renewed.  Ill  is  a  bit  of  detail  from  II,  still  more  en- 
larged, showing  two  capillaries  (Cap.)  conveying  corpuscles  (Corp.). 
The  air  is  close  by  (Al),  yet  two  partitions  intervene,  the  capillary 
wall  and  the  wall  of  the  air-sac. 


heart.  Other  tissues  have  a  minor  part  in  the  general  res- 
piration. Some  which  are  passive  and  stable,  like  cartilage, 
can  have  but  little. 

The  carbon  dioxid  formed  by  the  cells  is  first  transferred 
to  the  Ijmiph.  The  concentration  of  the  gas  in  the  lymph 
leads  to  its  passage  into  the  blood.  A  gas  will  always  pass 
from  a  higher  to  a  lower  concentration  when  the  two  solu- 
tions are  placed  in  communication.  The  delicate  capillary 
u 


162  NUTRITIONAL   PHYSIOLOGY 

wall  between  the  lymph  and  the  passing  blood  offers  prac- 
tically no  impediment  to  the  movement.  It  will  be  re- 
membered that  the  blood  which  enters  upon  the  very  short 
journey  through  the  capillaries  is  arterial;  that  which 
enters  the  minute  veins  only  a  fraction  of  an  inch  away  is 
reckoned  venous.  It  has  parted  with  a  large  share  of  its 
oxygen  and  has  received  carbon  dioxid.  It  is  swept  on 
without  significant  change  to  the  right  side  of  the  heart 
and  thence  to  the  lungs.  The  development  of  these  organs 
is  such  as  to  multiply  the  surface  of  contact  between  the 
blood  and  the  air  within  them.  The  capillaries  of  the 
pulmonary  circulation  are  wrapped  about  innumerable 
elastic  sacs,  the  walls  of  which  are  as  thin  as  those  of  the 
capillaries  themselves.  There  is,  accordingly,  a  double 
partition  between  the  blood  and  the  air,  but  it  is  of  a  nature 
to  permit  free  gaseous  exchange. 

If  the  air  in  the  sacs  were  not  renewed  it  would  accumu- 
late carbon  dioxid  in  increasing  amount,  while  its  oxygen 
would  progressively  diminish.  This  tendency  is  normally 
counteracted  through  the  effects  of  breathing.  The  lungs 
have  no  power  to  move  of  themselves;  the  changes  which 
they  undergo  are  due  to  the  widening  and  the  return  of  the 
thoracic  walls.  This  is  not  the  place  to  analyze  the  breath- 
ing movements.  The  muscles  employed  are  of  the  skeletal 
order.  Being  so,  they  are  not  automatic,  and  it  follows 
that  every  breath  taken  is  the  expression  of  a  separate  and 
distinct  act  on  the  part  of  the  central  nervous  system. 
Each  time  the  chest  is  made  larger  the  air  presses  in  along 
the  breathing  passages  to  fill  the  space  created  for  it  in 
the  host  of  widened  sacs.  The  return  of  the  chest  walls 
to  their  first  position  reduces  the  capacity  of  the  sacs,  and 
air  is  pressed  out  along  the  same  channels  by  which  it 
entered.  The  action  is  that  of  a  pair  of  bellows  not  pro- 
vided with  the  usual  inlet  valve. 

It  is  not  to  be  conceived  that  we  empty  and  refill  the 
air  spaces  of  the  lungs  Avith  each  breath.  We  usually  expel 
something  like  one-fifth  or  one-sixth  of  the  air  contained 
and  replace  that  fraction  with  fresh  air.     When  allowance 


REMOVAL    OF    THE    END-PRODUCTS    OF    METABOLISM     163 

is  made  for  the  rather  long  and  capacious  passages  between 
the  air-sacs  and  the  nostrils  the  impression  that  out  breath- 
ing is  rather  ineffective  becomes  strengthened.  To  offset 
this  idea  we  must  remember  that  the  movements  occur 
fifteen  or  eighteen  times  a  minute,  providing  thus  for  at 
least  two  fairly  complete  renewals  of  the  whole  volume  of 
air  within  that  time.  Still  it  is  a  fact  that  when  the  breath- 
ing is  rarely  deepened  beyond  the  constant  habit,  some 
portions  of  the  lungs,  notably  their  upper  extremities,  are 
but  little  subject  to  extension  and  contraction.  By  the 
deepest  possible  breathing  we  can  increase  the  proportion 
of  the  air  removed  and  replaced  to  perhaps  three-fourths  of 
the  total  at  a  single  movement.  A  quiet  breath,  unmodi- 
fied by  the  influence  of  attention,  muscular  activity,  or 
any  other  temporary  condition,  is  said  to  amount  to  about 
500  c.c.  (30  cubic  inches  or  1  pint).  Reckoning  sixteen 
breaths  in  a  minute,  this  will  mean  8  liters  of  air  breathed 
in  that  interval,  about  500  in  an  hour,  or  12,000  in  a  day. 
Fresh  air  has  but  a  small  content  (0.03  to  0.04  per  cent.)  of 
carbon  dioxid.  The  air  expired  has  4  per  cent.,  more  or 
less;  4  per  cent,  of  12,000  liters  is  480  liters,  a  fair  average 
volume  to  represent  the  daily  output  of  this  gas.  This 
quantity,  changed  from  volume  to  weight  with  correction 
for  temperature,  is  about  800  grams. 

The  air  to  which  the  blood  is  exposed  in  the  lungs  is  at 
least  as  rich  in  carbon  dioxid  as  that  which  we  breathe  out. 
Coming  into  relation  with  air  of  such  a  composition  the 
blood  by  no  means  frees  itself  of  its  large  carbon  dioxid 
content.  It  carries  on  to  the  left  side  of  the  heart  and  so  to 
the  general  arterial  system  some  five-sixths  of  the  carbon 
dioxid  which  it  contained  when  it  entered  the  lungs.  The 
actual  amount  in  venous  blood  is  in  the  neighborhood  of 
45  in  100  c.c.  of  blood.  As  much  as  38  c.c.  in  100  will 
usually  remain  in  blood  which  is  counted  arterial  and  which 
carries  a  maximum  of  oxygen.  Carbon  dioxid  does  not 
interfere  with  the  capacity  of  the  blood  to  carry  oxygen, 
and  the  converse  is  equally  true.  It  may  be  well  to  state 
that  the  color  of  blood  varies  with  the  extent  to  which  the 


164  NUTRITIONAL   PHYSIOLOGY 

corpuscles  are  charged  with  oxygen  and  is  independent 
of  the  carbon  dioxid  present. 

While  there  is  no  question  of  the  propriety  of  calling 
carbon  dioxid  a  waste-product,  it  does  not  follow  that  the 
system  would  be  benefited  by  its  complete  removal. 
How  far  this  is  from  being  the  case  has  been  shown  by  the 
important  experiments  of  Yandell  Henderson.  He  has 
demonstrated  that  any  considerable  lowering  of  the  carbon 
dioxid  below  the  high  standard  noted  above  as  character- 
istic of  arterial  blood  results  in  marked  prostration,  often 
involving  the  suspension  of  breathing  and  perhaps  result- 
ing fatally.  The  inference  is  that  a  certain  concentration 
of  carbon  dioxid  is  a  desirable  source  of  stimulation  to  the 
nervous  system  and  especially  to  the  respiratory  center. 
The  intimate  connection  between  this  gas  and  breathing  is 
manifest  when  its  percentage  in  the  blood  is  ever  so  little 
increased.  A  noteworthy  deepening  of  the  respiration 
promptly  results.  Since  this  is  true  it  is  not  strange  that 
a  reduction  of  the  carbon  dioxid  should  cause  inhibition  of 
the  breathing  movements. 

Water  Elimination. — Water  leaves  the  body  by  all  the 
possible  excretory  routes.  Statements  regarding  the 
proportion  taking  this  or  that  course  can  have  but  little 
value,  so  great  is  the  variation  under  different  circum- 
stances. If  we  exclude  the  effects  of  exercise  and  of  un- 
usual temperatures  we  may  expect  to  find  somewhat  more 
than  half  the  whole  amount  to  be  removed  by  the  kidneys. 
The  daily  volume  of  the  urine  is  customarily  set  down  at 
1200  to  1500  c.c.  The  remaining  excretion  of  water  will 
be  almost  wholly  accounted  for  by  the  perspiration  and  by 
evaporation  from  the  breathing  passages.  Of  these  two, 
the  former  is  commonly  more  considerable.  Some  loss  of 
water  will  occur  in  the  feces,  but  normally  this  is  not  to  be 
compared  with  the  quantities  discharged  in  the  three  ways 
just  mentioned. 

When  the  external  temperature  is  high  the  water  passes 
in  increased  amounts  through  the  skin  and  the  perspira- 
tion may  greatly  exceed  the  urine.     The  urinary  secretion 


Fig.  22. — The  kidneys  and  the  urinary  bladder.  The  two  kidneys 
are  shown  within  an  outHne  which  suggests  the  body  cavity.  Their 
advantageous  connections  with  the  chief  artery  and  vein  of  the  sys- 
tem are  indicated.  Below  is  the  bladder  reached  by  the  two  ureters. 
These  vessels  enter  the  bladder  low  down  and  behind — not  at  the 
level,  where  they  disappear  from  the  figure. 


REMOVAL   OF   THE    END-PRODUCTS    OF   METABOLISM      165 

may  shrink  somewhat  or  may  be  kept  near  the  standard  as 
a  result  of  the  water  drinking  which  is  stimulated.  The 
output  of  water  from  the  skin  during  the  hot  weather  and 
also  during  muscular  activity  does  not  serve  primarily  as  a 
vehicle  of  waste,  but  rather  as  a  means  of  ridding  the  body 
of  heat.  This  matter  will  be  discussed  at  length  at  an- 
other time.  The  evaporation  from  the  respiratory  tract 
probably  varies  less  widely  than  the  perspiration.  The 
body  seems  bound  to  saturate  all  the  air  that  is  breathed, 
so  that  this  loss  increases  with  the  volume  of  breathing 
and  is  greater  when  the  air  is  dry  than  when  it  is  humid. 

The  Kidneys  and  the  Urine. — The  chief  significance  of 
the  kidneys  is  their  function  of  excreting  the  distinctive 
products  of  protein  metabolism.  A  secondary  service  is 
the  disposal  of  inorganic  salts.  The  two  glands  are  placed 
to  the  right  and  left  of  the  spinal  column  at  the  level  of  the 
lower  ribs.  Each  kidney  receives  a  large  short  artery  from 
the  aorta  which  passes  between  them.  Each  returns  a 
large  vein,  not  to  the  portal  system,  but  to  the  chief  venous 
trunk  of  the  body.  The  kidneys,  in  consequence  of  this 
arrangement,  constitute  short  cuts  or  "shunts"  in  the  cir- 
culation, and  are  perfused  by  an  exceptionally  large  quan- 
tity of  blood.  No  portion  of  the  blood  can  long  escape 
their  influence.  The  urine  discharged  by  the  many  tubu- 
lar units  of  the  kidneys  is  conveyed  through  the  ureters, 
contractile  vessels  which  lead  to  the  bladder.  This  is  a 
saccular  organ  capable  of  accommodating  much  urine  when 
dilated,  and  of  contracting  again  to  nearly  complete  expul- 
sion of  its  contents.  Its  walls  of  muscle  (the  same  type 
found  in  the  alimentary  canal)  are  obviously  under  ner- 
vous control  and  much  subject  to  reflexes. 

Urine  of  average  composition  is  a  complex  solution  con- 
taining some  3  or  4  per  cent,  of  dissolved  solids.  The 
leading  substance  is  urea,  the  chief  nitrogenous  waste  of 
the  system,  and  the  index,  according  to  Folin,  of  the 
exogenous  metabolism.  Its  origin  has  been  discussed. 
Evidently  the  duty  of  the  kidney  is  less  to  manufacture 
urea  than  to  select  and  remove  from  the  blood  the  urea 


166  NUTRITIONAL    PHYSIOLOGY 

originating  in  the  liver  and  elsewhere.  Second  in  abun- 
dance among  the  urinary  constituents  we  ordinarily  find 
the  mineral  salts.  The  quantity  of  these  depends  in  a 
large  measure  upon  the  amount  in  the  diet,  and  as  sodium 
chlorid  is  the  one  taken  most  freely,  so  it  will  generally  be 
the  principal  inorganic  compound  in  the  urine.  The 
chlorids  of  the  mixture  are  accompanied  by  phosphates  and 
sulphates.  These  are  not  to  any  extent  salts  which  have 
been  eaten,  but,  like  the  urea,  they  represent  modified 
fragments  of  protein  molecules.  The  phosphates  come 
from  a  limited  class  of  proteins,  largely  from  those  of  the 
cell-nuclei;  the  sulphates  arise  from  all  proteins. 

The  minor  ingredients  of  the  urine  are  very  numerous. 
Those  of  most  interest  are  the  bodies  which  carry  the  ni- 
trogenous waste  over  and  above  that  handled  as  urea. 
To  one  unfamiliar  with  organic  chemistry  a  list  of  their 
names  can  have  little  meaning.  The  substance  creatinin, 
already  mentioned,  is  provisionally  regarded  as  indicative 
of  the  rate  of  true  endogenous  metabolism,  the  inevitable 
gradual  wasting  of  the  nitrogenous  tissues.  Another  com- 
pound which  has  attracted  much  attention  on  account  of  its 
apparent  relation  to  several  pathologic  conditions  is  uric 
acid.  A  certain  amount  of  this  is  produced  during  fasting 
and  is  not  increased  by  the  taking  of  many  kinds  of  food. 
The  addition  to  the  diet  of  meats  leads  to  a  larger  formation 
of  uric  acid.  A  maximum  quantity  is  elaborated  when 
glandular  tissues,  such  as  liver,  kidneys,  and  sweetbreads, 
are  eaten.  These  articles  contain  an  exceptional  propor- 
tion of  nuclear  material  rich  in  the  proteins  just  referred 
to  as  sources  of  phosphates.  The  same  proteins  are  evi- 
dently uric-acid  formers.  The  chief  peculiarity  of  uric 
acid  is  its  slight  solubility,  which  renders  its  complete 
excretion  difficult  and  uncertain.  Retention  of  this  crys- 
talline substance  has  been  held  accountable  for  the  pain- 
ful symptoms  of  gout  and  of  a  good  deal  that  goes  by  the 
inclusive  name  of  rheumatism. 

We  have  by  no  means  exhausted  the  list  of  normal  urin- 
ary constituents,  but  the  student  must  be  referred  to  other 


REMOVAL    OF   THE    END-PRODUCTS    OF   METABOLISM     167 

sources  for  details.  The  most  commonly  occurring 
compounds  of  an  abnormal  character  are  sugar  and  albu- 
min. The  significance  of  the  sugar  should  be  clear  in  the 
light  of  what  has  been  said.  Its  transient  appearance  as 
a  result  of  free  consumption  does  not  indicate  a  diseased 
condition,  but  only  dietetic  indiscretion.  Continuous  elim- 
ination shows  that  the  body  has  not  the  usual  power  to 
oxidize  its  sugar.  The  kidneys  are  not  generally  at  fault; 
the  defect  is  in  the  metabolism  of  the  pancreas  or  elsewhere. 
An  abundant  escape  of  albumin  (presumably  drawing 
upon  the  protein  mass  of  the  blood)  is  commonly  due  to  a 
disordered  condition  of  the  kidneys.  It  takes  place,  for 
example,  in  Bright's  disease. 

Urine  when  freshly  secreted  is  ordinarily  acid  to  litmus. 
It  may  be  alkaline  when  much  vegetable  food  is  eaten,  and 
becomes  so  on  standing  in  any  case.  The  change  is  due 
to  a  bacterial  fermentation  whereby  urea  is  transformed 
into  ammonium  carbonate.  An  ammoniacal  odor  develops 
in  connection  with  this  alteration  and  the  liquid  is  likely 
to  become  turbid.  The  deposition  of  a  sediment  under 
such  conditions  is  no  cause  for  anxiety.  It  has  often  been 
represented  by  unscrupulous  quacks  to  be  a  serious  symp- 
tom. The  urine  of  the  herbivora,  which  is  normally  al- 
kaline, becomes  acid  when  the  animals  are  fasting,  and  it 
may  be  pointed  out  that  they  are  then  carnivorous — living 
upon  their  own  flesh  and  fat. 

How  much  urine  is  secreted  depends  largely  on  the  quan- 
tity of  water  taken,  so  far  at  least  as  this  is  in  excess  of  the 
perspiration.  Kidney  activity  is  stimulated  by  almost 
any  dissolved  substance  foreign  to  the  standard  composi- 
tion of  the  blood.  The  nitrates,  for  example,  are  absorbed 
rather  freely  from  the  intestine  and  afterward  removed  by 
the  kidneys  in  a  large  volume  of  water.  They,  therefore, 
belong  to  the  class  of  bodies  known  as  diuretics.  The 
active  principle  of  coffee  and  tea  has  the  same  action.  In 
securing  their  own  elimination  such  compounds  may 
promote  the  excretion  of  others.  Diuretics  bring  about 
their  effect  partly  through  modifying  the  circulation,  and 


168  NUTRITIONAL   PHYSIOLOGY 

partly,  it  is  believed,  through  direct  influence  upon  the 
kidney  cells.  As  regards  the  first  mode  of  working,  it  may 
be  said  that  the  kidney  is  readily  responsive  to  increased 
blood-flow,  especially  if  it  is  attended  with  high  arterial 
pressure.  Difficulties  with  the  heart,  if  they  entail  re- 
tarded circulation  and  lowered  pressure,  frequently  lead  to 
deficient  output. 

Other  Factors  in  Excretion. — The  lungs  and  the  kidneys 
perform  so  large  a  share  in  the  disposal  of  metabolic  waste 
as  to  leave  relatively  little  of  the  work  undone.  The 
feces,  however,  include  small  quantities  of  miscellaneous 
excretions,  and  it  is  assumed  that  the  precise  part  borne  by 
the  intestine  and  the  liver  (in  the  separation  of  bile)  could 
not  be  taken  by  the  kidneys.  The  modified  bile-pigments 
and  cholesterin  of  the  feces  illustrate  this  specific  action. 
The  share  of  the  skin  in  the  removal  of  waste  is  popularly 
overestimated.  The  belief  that  "the  pores  must  be  kept 
open"  lest  poisons  gather  in  the  system  is  so  fruitful  of 
wholesome  practices  that  one  is  reluctant  to  question  it. 
Candor  requires,  however,  that  the  physiologist  repudiate 
the  moral  in  the  story  of  the  Italian  boy,  who  died  because 
the  surface  of  his  body  had  been  sealed  with  gold  paint 
for  a  few  hours.  If  the  case  is  authentic,  he  must  have 
died  because  of  the  character  of  the  application  and  not 
from  toxic  products  of  his  own  evolving.  Volunteers  have 
submitted  to  have  the  skin  shellacked  and  have  not  suf- 
fered any  other  ill  effects  than  sensitiveness  to  heat  and  cold. 

Perspiration  is  almost  purely  a  mineral  solution  and  the 
salts  it  carries  could  doubtless  be  cared  for  by  the  kidneys. 
When  made  profuse  by  severe  exercise,  it  contains  in  small 
amounts  some  of  the  organic  constituents  of  the  urine, 
but  its  highest  possible  rating  as  a  vehicle  of  nitrogen  ex- 
cretion is  not  impressive.  The  same  may  be  said  of  the 
assistance  rendered  by  the  skin  to  the  lungs.  Carbon 
dioxid  passes  from  the  skin  in  measurable  quantities 
when  there  is  abundant  perspiration,  but  the  largest  loss 
which  can  occur  in  this  way  seems  insignificant  when  com- 
pared with  the  discharge  from  the  lungs. 


CHAPTER  XVIII 
THE  ESTIMATION   OF  METABOLISM 

It  is  about  fifty  years  since  the  first  well-equipped 
laboratory  for  the  quantitative  study  of  human  nutrition 
was  opened  in  Munich.  Before  that  time  much  had  been 
accomplished  in  the  analysis  of  foods,  the  measurement  of 
rations,  and  the  examination  of  urine,  but  no  satisfactory 
knowledge  of  the  general  metabolism  could  be  had  until 
means  should  be  devised  to  entrap  and  measure  the  gas- 
eous outgo  of  the  body.  This  difficult  task  was  accom- 
plished by  the  construction  of  the  first  respiration  chamber, 
now  one  of  several  in  various  centers  of  scientific  research. 

In  the  long  run  there  must  be  a  correspondence  between 
the  food  and  the  metabolism,  the  income  and  the  outgo, 
but  on  a  single  day  there  is  no  necessary  agreement  be- 
tween them.  This  is  radically  demonstrated  on  a  day  of 
fasting,  when  the  income  is  nil  and  the  outgo  is  consider- 
able. It  is  to  the  excreta  that  we  attend,  therefore,  when 
we  wish  to  judge  to  what  extent  various  materials  have 
been  broken  down  in  the  body.  Studies  of  the  food  may 
be  valuable,  but  in  our  first  discussion  we  shall  limit  our- 
selves to  the  simple  case  of  the  subject  without  income. 
A  great  deal  can  be  learned  about  the  metabolism  by  deter- 
mining two  chemical  elements — the  carbon  and  the  nitro- 
gen of  the  waste-products.  Other  facts  can  be  ascertained 
when  the  quantity  of  oxygen  consumed  is  noted.  Water 
excretion  is  frequently  measured  also. 

Nitrogen  Elimination. — The  fact  is  already  familiar 
that-  the  nitrogen  leaving  the  system  is  found  almost 
wholly  in  the  urine.  An  additional  fraction  is  in  the 
feces.  Concerning  this  latter  item  it  will  be  remembered 
that  we  cannot  easily  say  how  largely  it  is  a  residue  signi- 

169 


170  NUTRITIONAL    PHYSIOLOGY 

fying  incomplete  absorption  and  how  far  it  is  a  true  waste- 
product.  If  feces  are  discharged  during  long  fasting  the 
nitrogen  contained  must  be  the  body's  own  contribution. 
On  the  whole,  the  fecal  nitrogen  is  nowadays  regarded  as 
an  excretion  unless  it  is  clearly  excessive.  The  nitrogen 
of  the  perspiration  can  usually  be  ignored. 

When  we  assume  all  the  nitrogen  eliminated  to  have 
come  from  the  decomposition  of  proteins,  a  certain  error 
always  exists,  but  it  is  not  great  enough  to  be  considered 
in  the  present  elementary  treatment  of  the  subject. 
Nitrogen  constitutes  about  16  per  cent,  of  protein.  Ac- 
cordingly, the  excretion  of  16  grams  is  taken  to  stand  for 
the  destruction  of  100  grams  of  protein.  This  is  not  far 
from  an  average  amount  when  the  diet  is  freely  chosen. 
In  the  opinion  of  an  increasing  number  of  authorities  it 
is  higher  than  it  should  be  for  the  best  nutritional  condi- 
tion. To  arrive  at  an  estimate  of  the  protein  metabolized 
we  multiply  the  quantity  of  nitrogen  in  the  outgo  by  6.25 
(an  operation  which  is  equivalent  to  dividing  by  16  to 
find  1  per  cent,  and  multiplying  by  100  to  obtain  the  total). 
This  was  a  familiar  procedure  before  the  erection  of  res- 
piration chambers  made  possible  a  complete  survey  of  the 
metabolism. 

Carbon  Elimination. — A  subject  excreting  16  grams  of 
nitrogen  may  be  expected  to  excrete  something  like  200  or 
250  grams  of  carbon  in  the  same  period.  This  will  be  in 
the  respiratory  carbon  dioxid  so  largely  as  to  make  the 
urinary  and  fecal  carbon  appear  insignificant.  Figures 
from  an  actual  experiment  are: 

In  the  respiration 208  grams. 

In  the  urine 6       " 

In  the  feces 11       " 

Total 225       " 

This  carbon  may  have  been  furnished  by  all  three  types  of 
body  substance — the  proteins,  fats,  and  carbohydrates — 
in  numberless  possible  combinations.  The  amount  of 
protein  decomposition  has  already  been  fixed  at  100  grams. 


THE    ESTIMATION    OF    METABOLISM  171 

Such  an  amount  of  protein  must  have  yielded  in  its  faUing 
apart  a  quantity  of  carbon  represented  by  the  percentage 
of  that  element  in  the  compound.  The  actual  percentage 
is  about  53,  so  that  in  this  instance  53  grams  of  the  225 
may  be  ascribed  to  protein  as  a  source.  The  remaining 
carbon,  172  grams,  must  have  been  derived  from  non- 
nitrogenous  material.  How  far  it  has  come  from  fat  and 
how  far  from  carbohydrate  we  cannot  exactly  determine 
without  additional  data. 

It  is  of  value  to  know  all  the  circumstances  of  such  a 
trial.  If  the  twenty-four  hours  under  consideration  is  the 
first  fasting  day  and  the  diet  of  the  day  before  has  been  the 
ordinary  one,  we  may  assume  that  the  subject  entered 
upon  the  experimental  period  with  a  fair  stock  of  glycogen. 
This  will  be  used  rather  freely  at  the  outset,  but  more 
and  more  slowly  as  the  hours  pass.  Carbohydrate,  accord- 
ingly, contributes  largely  to  the  support  of  the  organism 
during  the  first  day  of  abstinence,  and  thereafter  bears  but 
a  very  small  part.  To  say  that  the  glycogen  of  the  body 
is  used  up  in  a  single  day  of  fasting  would  not  be  correct; 
the  fact  is  rather  that  the  rate  of  consumption  diminishes 
sharply.  In  proportion  to  this  diminution  in  the  use  of 
carbohydrate  the  fat  is  called  upon  increasingly.  For  any 
day  of  hunger  after  the  first  it  is  substantially  true  that 
the  individual  is  living  on  protein  and  fat. 

Let  us  continue  the  discussion  of  our  numerical  illus- 
tration with  the  added  statement  that  the  day  is  the  second 
rather  than  the  first  in  a  fast.  The  172  grams  of  carbon 
from  non-protein  material  may  now  be  attributed  to  fat. 
The  percentage  of  carbon  in  fat  is  about  77.  A  simple  cal- 
culation (dividing  by  77  and  multiplying  by  100,  or,  what 
is  the  same  thing,  multiplying  our  first  quantity  by  1.3) 
gives  223.6  grams  of  fat  as  the  amount  destroyed  in  the 
body  during  twenty-four  hours.  The  total  metabolism 
is  then  100  grams  of  protein  and  223.6  grams  of  fat.  It  is 
not  safe  to  conclude  from  this  that  the  loss  of  weight 
will  prove  to  be  just  equal  to  the  sum  of  the  two  items.  It 
may  be  found  to  be  either  more  or  less,  the  result  depend- 


172  NUTRITIONAL   PHYSIOLOGY 

ing  chiefly  on  the  relation  of  the  income  and  outgo  of 
water. 

Could  there  be  conditions  under  which  all  the  non-pro- 
tein carbon  could  confidently  be  assigned  to  carbohydrate 
sources?  Not  in  the  fasting  state  nor  commonly  on  a 
mixed  diet.  The  case  might  be  approximated  by  giving 
ample  rations  with  minimal  fat  and  maximal  carbohydrate 
for  days  together.  This  is  nearly  equivalent  to  the  nutri- 
tion of  the  herbivora.  If  our  supposed  human  subject 
yielded  the  amounts  of  carbon  and  nitrogen  already 
quoted  while  adequately  fed  upon  protein  and  carbo- 
hydrate, we  should  not  be  much  in  error  in  assuming 
that  the  carbon  from  non-protein  had  been  evolved  from 
starches  and  sugars  metabolized.  Carbon  forms  about  40 
per  cent,  of  carbohydrate,  and,  if  we  reckon  according  to 
the  same  principle  as  before,  we  find  that  172  grams  of 
carbon  could  have  come  from  430  grams  of  carbohydrate, 
more  or  less.  Under  more  ordinary  conditions  of  feeding 
— and  on  the  first  day  of  a  fast — both  carbohydrate  and 
fat  would  share  with  protein  the  sustaining  of  the  body's 
activities. 

The  Respiratory  Quotient. — The  modern  respiration 
chamber  is  a  small  room  with  impervious  walls  and  care- 
fully controlled  ventilation.  The  carbon  dioxid  of  the 
air  drawn  off  is  either  determined  directly  or  estimated 
from  measured  samples  bearing  a  known  relation  to  the 
total  volume.  In  chambers  of  the  best  type  the  oxygen 
consumption  is  also  ascertained.  If  we  know  both  the 
carbon  dioxid  production  and  the  oxygen  absorption  we 
can,  of  course,  compute  the  ratio  between  the  two.  The 
value  of  this  ratio,  based  on  the  volumes  and  not  the 
weights  of  the  two  gases,  is  known  as  the  respiratory  quo- 
tient. It  is  figured  by  dividing  the  volume  of  carbon  dioxid 
by  the  volume  of  oxygen.  So  determined  it  has  most  of  the 
time  the  character  of  a  proper  fraction,  or,  rendered  as  a 
decimal,  it  is  less  than  one.  This  is  a  way  of  saying  that 
the  carbon  dioxid  discharged  is  generally  less  than  the 
oxygen  which  has  disappeared  in  the  exchange. 


THE    ESTIMATION    OF   METABOLISM  173 

Every  molecule  of  carbon  dioxid  holds  combined  the 
equivalent  of  a  molecule  of  oxygen.  It  follows  that  if  all 
the  oxygen  were  devoted  to  the  formation  of  carbon  dioxid 
the  two  volumes  would  be  equal  and  the  ratio  between 
them  would  be  unity.  The  failure  of  a  part  of  the  oxygen 
to  reappear  as  carbon  dioxid  indicates  that  it  has  been 
combined  in  some  other  way.  It  has  actually  gone  to  form 
the  second  great  respiratory  product,  the  water  of  the 
metabolism.  The  interest  which  physiologists  feel  in  the 
respiratory  quotient  springs  from  the  fact  that  it  varies 
with  the  prevailing  employment  of  one  kind  of  material 
or  another  in  the  general  oxidation  which  is  going  on. 
The  decimal  value  of  the  ratio  is  elevated  in  proportion 
to  the  prominence  of  carbohydrate  in  the  process.  It  is 
lowest  when  fat  is  bearing  a  principal  part.  Since,  at  the 
beginning  of  a  fast,  carbohydrate  is  called  upon  to  meet 
the  requirement,  while  its  place  is  taken  by  fat  a  few  hours 
later,  the  respiratory  quotient  will  show  a  decline  which 
marks  with  exactness  the  shifting  of  the  current. 

It  would  not  be  easy  to  show  how  the  respiratory  quotient 
can  be  made  the  basis  of  equations  which  determine 
how  much  fat  and  how  much  carbohydrate  are  broken 
down  to  give  a  certain  output  of  carbon  dioxid.  Suffice  it 
to  say  that  the  possibility  exists  and  is  highly  fruitful  of 
results  in  the  quantitative  studies  of  nutrition  laboratories. 
The  meaning  of  the  respiratory  quotient  is  sometimes  al- 
tered by  temporary  conditions.  Perhaps  the  most  in- 
teresting of  these  is  the  peculiar  increase  in  carbon  dioxid 
outgo  exhibited  by  an  animal  which  is  rapidly  fattening  on 
a  diet  rich  in  carbohydrates.  Such  an  animal  may  show 
for  days  together  a  respiratory  quotient  in  excess  of  unity, 
that  is,  it  produces  carbon  dioxid  not  accounted  for  by  the 
observed  oxygen  intake.  This  extra  carbon  dioxid  is  ex- 
plained satisfactorily  as  having  come  from  the  carbo- 
hydrate undergoing  transformation  to  fat.  (See  also 
Chapter  XV,  page  143.) 

Equilibrium. — Our  nearly  uniform  weight,  maintained 
for  periods  of  years,  suggests  that  income  and  outgo  are 


174  NUTRITIONAL    PHYSIOLOGY 

often  nicely  balanced.  Complete  equilibrium  demands 
strict  equality  between  the  income  and  outgo  of  water,  of 
mineral  matter,  of  nitrogen,  and  of  carbon.  The  realiza- 
tion of  these  conditions  is  not  likely,  though  it  is  often 
closely  approached.  Partial  equilibrium,  that  is,  equality 
between  intake  and  output  for  one  class  of  compounds  with 
inequality  for  another,  is  more  common.  The  most  fre- 
quent striking  of  a  balance  is  between  the  nitrogen  of  the 
food  and  that  of  the  excreta.  Nitrogenous  equilibrium  is 
the  rule  rather  than  the  exception.  The  tendency  of  the 
body  to  establish  this  correspondence,  in  spite  of  wide 
variations  in  the  diet,  was  noted  long  ago.  It  was  said 
that  the  organism  refused  to  store  protein  when  supplied 
with  large  amounts  of  this  kind  of  food.  This  is  true  in  the 
narrow  sense  that  the  body  does  not  add  freely  to  the  adult 
measure  of  its  living  tissues  when  offered  extra  protein. 
Yet,  as  we  have  seen,  the  return  of  all  the  nitrogen  fed 
does  not  of  necessity  mean  that  the  body  has  retained  no 
part  of  the  protein  supplied  to  it.  The  chief  reason  why 
there  is  such  a  marked  disposition  to  make  the  output  of 
nitrogen  equal  the  income  is  found  in  the  fact  that  all  the 
amino-acids  beyond  the  small  quantity  used  for  protein 
synthesis  are  deaminized.  So  long  as  this  is  the  case, 
raising  the  nitrogen  of  the  food  must  result  merely  in 
adding  to  the  urea  excreted.  The  non-nitrogenous  resi- 
dues may  find  more  or  less  permanent  lodgment  in  the 
tissues  in  the  form  of  glycogen  or  fat. 

Nitrogen  retention  is  to  be  expected  during  growth  when 
the  protein  syntheses  are  more  extensive  than  the  endogen- 
ous decomposition.  The  recovery  from  illness  or  from  a  fast 
is  another  instance  when  the  body  protein  must  definitely 
increase.  This  is  really  only  a  special  case  of  growth. 
Change  of  climate  or  the  pursuit  of  athletic  training  may 
encourage  some  degree  of  protein  storage.  And  without 
seriously  qualifying  what  has  just  been  said  it  may  be 
stated  that  abundant  nitrogenous  food  may  have  the  same 
effect,  but  the  nitrogen  retention  secured  by  forced  feeding  is 
always  limited  to  a  very  small  fraction  of  the  protein  given. 


THE    ESTIMATION    OF    METABOLISM  175 

The  roughly  maintained  equihbrium,  which  is,  after  all, 
a  striking  example  of  the  adjustments  of  the  organism,  is 
to  be  traced  to  the  singular  reliability  of  the  appetite. 
This  is  the  agent  which  prompts  so  surely  to  the  taking  of 
extra  food  when  one  exchanges  an  inactive  life  for  one  of 
bodily  activity.  The  most  radical  changes  in  the  total 
metabolism  are  unlikely  to  lead  to  lasting  variations  in 
body  weight  beyond  slight  gains  and  losses,  which,  by  the 
way,  are  often  the  reverse  of  what  was  anticipated.  Ex- 
ercise, which  is  supported  by  large  oxidation,  may  even 
result  in  some  increase  of  weight,  showing  that  the  appe- 
tite has  rather  more  than  met  the  precise  need  of  the  body. 

Carbon  Retention. — When  a  quantitative  comparison 
is  made  between  the  compounds  in  the  diet  and  those  ex- 
creted it  is  not  infrequently  found  that  carbon  is  being 
stored,  though  the  nitrogen  of  income  and  outgo  may  be 
balanced.  What  can  be  inferred  as  to  the  nature  of  the 
substance  added  to  the  tissues?  Just  as  in  the  previous 
case  where  we  desired  to  interpret  the  meaning  of  the 
carbon  loss  during  fasting,  we  have  to  consider  the  re- 
spective share  taken  by  carbohydrate  and  by  fat.  As  be- 
fore, it  is  important  to  know  the  condition  of  the  subject 
prior  to  the  trial.  If  the  day  is  the  first  of  feeding  after 
a  fast  there  will  be  some  recruiting  of  the  glycogen  in  the 
body,  and  a  part  of  the  carbon  retention  may  be  attributed 
to  a  gain  of  this  material.  Otherwise,  when  carbon  is 
stored  in  the  midst  of  a  period  of  liberal  feeding,  the  prob- 
ability is  that  fat  rather  than  carbohydrate  has  been  de- 
posited. 

Applying  the  same  factors  as  in  the  earlier  instance,  we 
multiply  the  retained  carbon  by  1.3  if  the  circumstances 
point  to  its  having  been  held  as  fat.  An  excess  of  10 
grams  of  carbon  in  the  income  over  the  outgo  would  be 
assumed  to  indicate  the  addition  of  13  grams  of  fat  to  the 
supply  in  the  body.  An  amount  of  this  magnitude  would 
not  show  itself  decisively  in  the  weight,  being  easily  dis- 
guised by  the  temporary  gain  or  loss  of  water  that  might 
occur  at  the  same  time. 


CHAPTER  XIX 
THE  ENERGY   OF  THE  METABOLISM 

The  initial  statement  in  this  book — that  living  things 
are  transformers  of  matter  and  energy — ^is  a  text  to  which 
we  have  closely  adhered.  In  recent  chapters  the  emphasis 
has  been  placed  upon  transformations  of  matter.  We 
shall  now  pass  on  to  speak  of  the  energy  evolved  by  animals 
and  particularly  by  the  human  body.  The  fundamental 
facts  are  presumably  clear.  The  energy  of  the  income  is 
potential  in  the  complex  molecules  of  the  food.  It  is 
released  in  the  oxidative  decomposition  processes  of  life 
and  made  kinetic.  It  appears  chiefly — often  solely — 
in  the  form  of  heat.  Measurements  of  the  heat  production 
of  living  organisms  are  generally  to  be  accepted  as  indic- 
ative of  the  total  energy  production.  Certain  exceptions 
to  the  rule  will  soon  be  noted. 

Fuel  Values. — Since  energy  can  be  transmuted  from  one 
form  to  another  it  is  possible  to  make  the  units  which  stand 
primarily  for  one  kind  do  duty  for  all.  It  is  our  constant 
practice  to  use  the  units  of  heat  to  measure  all  the  energy 
of  metabolism.  The  unit  which  we  shall  employ  is  the 
large  Calorie,  approximately  defined  as  the  amount  of 
energy  required  to  raise  the  temperature  of  1  kilogram  of 
water  one  centigrade  degree.  The  large  Calorie  is  invari- 
ably distinguished  from  the  small  by  the  capital  C.  The 
small  calorie  is  ttott  of  the  large;  no  further  reference  to 
it  will  be  made.  When  a  combustible  organic  substance 
of  a  standard  composition  is  completely  oxidized  a  definite 
quantity  of  heat  is  evolved.  The  heat  produced  by 
oxidizing  1  gram  of  any  compound  is  its  Juel  value. 

The  highest  fuel  value  recorded  is  that  of  hydrogen, 
about  34  Cal.  This  is  the  amount  of  heat  produced  when 
176 


THE  ENERGY  OF  THE  METABOLISM       177 

a  gram  of  hydrogen  gas  (11  liters)  is  oxidized  to  water. 
One  gram  of  carbon  oxidized  to  carbon  dioxid  gives  nearly 
8  Cal.  These  two  illustrations  do  not  bear  directly  on  our 
physiologic  inquiry,  for  the  body  does  not  use  the  free 
elements  for  oxidation,  but  their  compounds.  It  is,  there- 
fore, of  more  interest  to  turn  to  the  fuel  values  of  carbo- 
hydrates, fats,  alcohol,  and  proteins,  since  these  are  the 
actual  sources  of  heat  and  kindred  energy.  The  calorific 
value  of  a  compound  is  not  precisely  that  of  the  carbon  and 
the  hydrogen  contained  in  it  and  not  yet  bonded  to  oxygen, 
although  some  early  work  of  a  useful  kind  was  based  upon 
that  assumption.  It  is  to  be  noted  that  the  oxygen  in  the 
physiologic  compounds  reduces  their  potency;  the  less  they 
contain  the  more  largely  they  will  consist  of  elements  sub- 
ject to  oxidation.  This  is  the  main  reason  why  fats  have 
fuel  values  greatly  in  excess  of  those  of  carbohydrates. 
A  gram  of  fat  contains  nearly  twice  as  much  carbon  as  a 
gram  of  sugar.  It  also  contains  much  hydrogen  with  un- 
satisfied afl&nities  for  oxygen. 

The  actual  heat  production  observed  when  a  gram  of 
starch  is  burned  is  a  trifle  more  than  4  Cal.  Sugars,  which 
are  slightly  richer  in  oxygen  than  starch  is,  have  a  little 
lower  fuel  value.  The  figure  (4  Cal.)  is  fairly  representa- 
tive of  carbohydrates  as  a  class.  The  oxidation  of  1 
gram  of  fat  liberates  about  9.3  Cal.,  or  2j  times  as  much  as 
starch.  A  gram  of  grain  alcohol  fully  oxidized  gives  an 
intermediate  quantity,  about  7  Cal.  All  these  non-nitrog- 
enous compounds  are  made  to  yield  the  same  simple  prod- 
ucts, namely,  carbon  dioxid  and  water,  whether  they  are 
destroyed  by  literal  burning  outside  the  body  or  by  the 
metabolic  processes.  We  shall  see  shortly  that  the  energy 
which  is  found  to  be  set  free  in  their  oxidation  can  be 
proved  to  be  equal  in  the  two  cases. 

Protein  stands  somewhat  apart  in  its  behavior.  A  gram 
of  dried  protein  burned  in  oxygen  gives  nearly  6  Cal. 
But  some  of  the  products  generated  in  such  a  laboratory 
test  are  not  those  which  the  body  forms  from  protein  and 
excretes.     Urea,  for  example,  is  not  foimd  after  the  actual 

12 


178  NUTRITIONAL    PHYSIOLOGY 

burning  of  protein.  Urea  itself  has  a  certain  capacity  for 
oxidation  and  a  low  but  distinct  fuel  value,  something 
like  2.5  Cal.  per  gram.  The  residual  fuel  value  in  urea  is  a 
sign  that  some  portion  of  the  energy  latent  in  protein  is 
constantly  lost  to  the  animal  economy.  Bacteria  may 
profit  by  it,  but  it  seems  not  to  be  available  for  the  higher 
living  forms.  There  are  certain  products  other  than  urea 
which  remain  after  the  decomposition  of  protein  molecules 
and  which  likewise  represent  unused  energy.  Some  of 
these  minor  products  accompany  the  urea  in  the  urine, 
while  others  are  mingled  with  the  feces.  To  make  ac- 
curate allowance  for  the  energy  lost  with  these  incompletely 
oxidized  compounds  is  a  difficult  matter.  The  estimate 
arrived  at  credits  to  a  gram  of  protein  4  Cal.  or  a  little 
more.  It  is  a  coincidence  with  very  convenient  results 
that  this  is  almost  exactly  the  same  as  the  figure  for  car- 
bohydrate. 

The  Total  Daily  Metabolism. — The  widest  limits  be- 
tween which  the  metabolism  of  an  adult  may  vary  may  be 
set  down  as  1000  and  10,000  Cal.  per  day.  The  lowest 
level  will  be  approached  when  there  is  complete  rest  and 
protection  from  cold  during  the  twenty-four  hours.  The 
maximum  will  be  reached,  if  ever,  when  a  large,  powerful 
man  performs  the  heaviest  muscular  work  while  under  ob- 
servation. The  food  taken  on  a  single  day  influences 
the  result  less  than  would  be  anticipated.  (As  already 
pointed  out,  the  appetite  follows  the  metabohsm  rather 
than  precedes  it.)  High  protein  feeding  has  an  effect 
which  will  be  discussed  later. 

Of  course,  the  average  for  an  individual  will  generally 
fluctuate  much  less  than  is  suggested  above.  It  will 
rarely  fall  below  1500  or  rise  above  3000  under  the  con- 
ditions of  city  life.  Until  recently  it  would  have  appeared 
reasonable  to  fix  upon  2400  as  a  mean.  This  value  has 
lately  come  to  be  regarded  as  higher  than  the  actual  energy 
output  of  most  people.  Perhaps  2000  may  be  adopted  as 
an  ordinary  amount.  For  sixteen  hours  of  waking  we 
may  allow  100  Cal.  per  hour,  and  for  eight  hours  of  sleep 


THE  ENERGY  OF  THE  METABOLISM       179 

60  per  hour,  an  estimate  adding  up  to  2080.  It  will  be 
suggestive  if  we  consider  how  much  of  a  single  food-stuff 
would  be  required  to  furnish  a  total  of  this  order. 

Two  thousand  Calories  could  be  obtained  by  the  oxida- 
tion of  about  500  grams  of  starch  or  sugar.  The  meeting 
of  the  energy  requirement  is  not  the  only  qualification  to 
be  demanded  of  a  day's  dietary.  A  ration  of  pure  sugar 
is  obviously  not  to  be  recommended,  though  the  consump- 
tion of  candy  to  such  an  extent  may  be  entirely  possible 
for  some  subjects.  The  quantity  of  fat  needed  to  afford 
the  same  heat  value  will  be  in  the  vicinity  of  216  grams. 
One  can  hardly  conceive  of  eating  clear  fat  to  this  amount. 
Making  a  similar  calculation  for  alcohol,  we  find  that  a  little 
less  than  300  grams  of  this  compound  will  theoretically 
meet  the  need.  A  palpable  absurdity  is  apparent.  While 
the  attempt  to  use  alcohol  exclusively  as  a  fuel  for  the 
body  would  evidently  be  disastrous,  it  is  interesting  to 
consider  that  a  lamp  burning  300  grams  of  absolute  alcohol 
in  a  day  would  equal  a  human  being  as  a  source  of  energy. 
The  flame  would  be  a  very  small  one  and  the  comparison 
gives  one  a  feeling  of  dissatisfaction  and  disappointment. 

Since  protein  is  nearly  equivalent  to  carbohydrate  as  a 
source  of  energy,  the  theoretic  ration  of  protein  would  be 
the  same  as  that  of  sugar,  namely,  500  grams.  The  prac- 
tical impossibility  of  eating  so  much  protein  is  evident 
when  it  is  remembered  that  protein  is  acceptable  only  when 
softened  and  expanded  with  liberal  quantities  of  water. 
Lean  meat,  which  is  not  strictly  a  straight  protein  food, 
but  which  approaches  that  composition,  is  three-fourths 
water.  White  of  egg,  in  which  the  solid  portion  is  almost 
all  protein,  is  seven-eighths  water.  The  attempt  to  dry 
such  material  and  so  to  reduce  it  to  the  smallest  bulk 
produces  a  mass  resembling  hardened  glue  or  shellac. 
To  subsist  on  the  proteins  of  meat  alone,  the  fat  having 
been  removed,  one  would  be  compelled  to  eat  some  five 
pounds  daily.  Travelers  have  described  certain  peoples  as 
living  almost  wholly  on  meat  or  fish,  but  this  does  not  mean 
a  pure  protein  diet,  the  fat  undoubtedly  figuring  largely. 


180  NUTRITIONAL   PHYSIOLOGY 

The  Eskimos  necessarily  eat  but  little  carbohydrate,  for 
they  can  obtain  no  vegetable  food  of  importance;  it  seems 
plain,  however,  that  they  have  increased  both  fat  and  pro- 
tein consumption  and  not  protein  alone. 

The  foregoing  suggestions  make  clear  the  inconveniences 
and  the  unhygienic  aspect  of  any  attempt  to  live  on  a 
single  type  of  food.  It  is  a  fact,  furthermore,  that  one 
could  not  under  any  conditions  continue  indefinitely  to  eat 
only  non-nitrogenous  food.  Protein  metabolism  never 
ceases  and  a  certain  nitrogenous  income  must  be  provided. 
We  cannot,  therefore,  judge  the  fitness  of  a  diet  solely  by 
its  heat  value.  It  must  measure  up  to  a  reasonable  stand- 
ard in  this  respect,  but  it  must  also  include  a  suitable  pro- 
portion of  protein.  Another  criterion,  not  so  commonly 
insisted  upon,  is  that  a  sufficient  quantity  and  variety  of 
mineral  compounds  shall  be  supplied.  Of  course,  these 
scientific  characterizations  of  the  diet  are  inadequate  unless 
attention  is  paid  also  to  attractiveness  and  digestibility. 

When  a  subject  freely  chooses  his  food,  unprejudiced 
by  chemical  knowledge,  he  is  apt  to  make  use  of  all  three 
classes  of  food-stuffs  to  a  considerable  extent.  Carbo- 
hydrate will  usually  amount  to  more  than  half  the  total 
solids  of  the  ration,  Avhile  protein  and  fat  show  a  curious 
tendency  to  be  taken  in  nearly  equal  weights.  An  ex- 
ample of  such  a  selection,  perhaps  the  one  most  frequently 
quoted,  is  the  following: 

Protein 100  grams    (410  Calories). 

Fat 100       "         930      " 

Carbohydrate 250       "        1025      " 

'2365      " 

Of  late  years,  observation  having  been  extended  to  large 
numbers  of  people,  it  has  become  evident  that  Americans 
of  student  and  professional  classes  rarely  choose  to  eat  as 
much  as  100  grams  of  protein. 

Calorimetry. — We  have  been  speaking  of  the  amounts 
of  heat  set  free  in  the  oxidation  of  various  food  materials, 
and  of  the  energy  liberated  by  animals  and  men  as  an  ac- 


THE  ENERGY  OF  THE  METABOLISM       181 

companiment  of  their  metabolism.  The  determination  of 
such  data  requires  the  use  of  calorimeters.  These  are  of 
various  forms,  but  have  the  same  underlying  principle.  In 
case  a  food  sample  is  to  be  burned  it  is  enclosed  in  a 
small  chamber,  preferably  in  an  atmosphere  of  oxygen,  and 
its  combustion  is  initiated  by  means  of  an  electric  spark. 
The  heat  is  imparted  to  a  large  mass  of  water  surrounding 
the  chamber  and  can  then  be  estimated  by  the  elevation  of 
temperature  observed  in  this  water.  This  is  an  elementary 
and  somewhat  sweeping  statement;  numerous  corrections 
would  be  necessary  in  practice. 

One  of  the  earliest  attempts  to  gage  the  heat  production 
of  an  animal  consisted  in  confining  it  within  a  chamber 
having  double  walls  and  ice  between.  The  amount  of  ice 
melted  could  thus  be  made  the  basis  for  the  estimate  de- 
sired. The  employment  of  a  water  calorimeter  evidently 
does  away  with  the  unnatural  chilling  which  must  have 
been  entailed  in  this  primitive  trial.  The  large  and  costly 
calorimeters  in  modern  laboratories  for  the  study  of  human 
nutrition  are  of  the  water  type.  No  adequate  idea  of  the 
difficulty  involved  in  such  work  is  likely  to  be  grasped  by 
reading  this  condensed  presentation.  Ventilation  must  be 
maintained  and  allowance  made  for  the  warming  of  the 
circulating  air.  The  evaporation  of  water  must  be  ac- 
curately measured,  for  if  this  were  ignored  a  great  deal  of 
beat  produced  in  the  metabolism  would  escape  measure- 
ment. (In  the  evaporation  of  1  kilogram  of  water  heat  to 
the  amount  of  536.5  Cal.  will  be  made  latent.)  Changes 
in  the  body  temperature  cannot  be  overlooked.  If,  for 
example,  a  human  subject  equivalent  in  heat  capacity  to 
50  kilograms  of  water  should  experience  a  rise  of  tempera- 
ture amounting  to  0.5°  C.  during  an  experiment,  25  Cal. 
would  have  been  retained  in  his  body.  If,  instead,  there 
were  a  fall  of  0.5  degrees  there  would  have  been  a  discharge 
of  25  Cal.,  and  the  observed  heat  output  would  have  been 
greater  than  that  due  to  the  current  metabolism. 

Now  the  question  may  be  asked  whether  we  can  be  sure 
that  the  observed  heat  production  (corrected  for  body 


182  NUTRITIONAL   PHYSIOLOGY 

temperature  changes  and  supplemented  by  the  heat  loss 
through  evaporation)  is  a  just  representation  of  the  total 
energy  set  free.  May  there  not  be  other  forms  of  energy 
than  heat?  What  are  the  facts  concerning  muscular 
work?  We  cannot  say  positively  that  no  energy  eludes 
the  calorimeter,  but  the  impression  is  constantly  being 
confirmed  that  any  such  escape  must  be  insignificant.  As 
to  the  energy  of  movement  it  can  be  shown  that  under  most 
conditions  this  will  eventuate  in  heat.  This  is  an  import- 
ant matter  and  deserves  to  be  carefully  illustrated. 

Let  us  take  for  an  example  the  case  of  the  heart.  This 
organ  does  a  great  deal  of  work  in  forcing  the  blood  through 
the  vessels.  Does  this  work  appear  as  heat  so  as  to  be 
arrested  and  recorded  by  the  calorimeter?  There  is  no 
doubt  that  it  does.  The  conversion  is  effected  as  the  re- 
sistance to  the  blood-flow  is  encountered  and  overcome. 
The  heart  produces  some  heat  within  itself,  and  some  addi- 
tional heat  due  to  its  metabolism  is  made  to  appear  in  all 
parts  of  the  body.  Wherever  the  blood  is  driven  there  is 
friction,  which  is  a  means  of  transforming  the  energy  of 
motion  into  heat.  The  same  statement  applies  to  the 
breathing  movements.  Work  is  done,  in  the  physical  sense 
of  the  term,  each  time  the  ribs  are  lifted,  but  with  their 
return  to  the  expiratory  position  this  work  is  reconverted 
into  heat.  So,  too,  all  ordinary  forms  of  exercise  may  be 
shown  to  result  in  heat  production. 

Nevertheless  it  is  possible  to  devise  conditions  under 
which  a  part  of  the  metabolic  energy  will  not  be  given  to 
the  calorimeter.  Suppose,  for  example,  that  the  subject 
within  the  apparatus  is  employed  in  taking  books  from  the 
floor  and  placing  them  upon  shelves.  As  long  as  he  pur- 
sues this  form  of  activity  a  share  of  his  evolved  energy  will 
become  potential  and  be  lost  to  direct  observation.  It 
may  be  said  to  exist  as  "energy  of  position''  in  the  mass 
which  he  has  elevated.  In  other  words,  it  has  been  stored. 
If,  after  a  day  of  this  labor,  he  should  occupy  himself  in 
removing  the  books  from  the  shelves  and  laying  them  upon 
the  floor,  he  would  be  giving  to  the  calorimeter  more  heat 


THE  ENERGY  OF  THE  METABOLISM       183 

than  that  actually  produced  as  a  result  of  his  metabolism. 
When  the  books  had  all  l^een  returned  to  the  original  level 
the  sum  of  the  calories  for  the  two  periods  would  justly 
represent  the  energy  production  of  his  body.  One  could 
conceive  of  two  calorimeters  placed  side  by  side,  in  one  of 
which  a  man  might  turn  a  crank  operating  a  shaft  which 
should  pass  into  the  second  chamber  and  there  revolve  a 
wheel  against  the  resistance  of  a  brake.  Most  of  his  energy 
would  be  registered  by  the  first  calorimeter,  but  a  fair  pro- 
portion of  it,  standing  for  most  of  his  muscular  work,  would 
be  apparent  in  the  second. 

Direct  and  Indirect  Calorimetry. — We  have  said  else- 
where that  the  proof  of  the  validity  of  the  principle  of  the 
conservation  of  energy  for  living  things  was  a  great  achieve- 
ment of  the  nineteenth  century  physiologists.  The 
method  of  this  proof  may  now  be  outlined.  We  have  just 
seen  that  the  total  energy  production  of  animals,  including 
man,  may  be  satisfactorily  measured  in  calorimeters, 
provided  that  the  amount  of  evaporation  is  known  and 
any  changes  of  body  temperature  considered.  This  pro- 
cedure is  direct  calorimetry.  Now,  of  course,  it  is  possible, 
during  the  same  period  to  collect  the  excreta  and  to  deter- 
mine the  character  and  the  amount  of  the  metabolism 
according  to  the  principles  explained  in  the  last  chapter. 
Knowing  the  metabolism — so  many  grams  of  protein,  of 
fat,  and  of  glycogen  destroyed — we  may  credit  to  each  of 
these  its  respective  fuel  value  and  calculate  the  number  of 
calories  which  could  theoretically  result  from  just  such 
oxidation.  We  may  then  compare  the  energy  as  deter- 
mined from  the  living  organism  and  the  energy  which 
should  have  been  liberated  in  the  formation  of  the  measured 
wastes. 

To  make  the  matter  plain,  we  may  refer  once  more  to  the 
case  on  which  the  numerical  estimation  of  metabolism 
was  previously  based.  The  subject  was  credited  with  a 
metabolism  of  100  grams  of  protein  and  223.6  grams  of  fat. 
(This,  it  may  be  recalled,  is  the  usual  assumption  when 
the  day  is  one  of  fasting  and  does  not  closely  follow  carbo- 


184  NUTRITIONAL    PHYSIOLOGY 

hydrate  feeding.)  The  apparent  heat  value  of  the  calcu- 
lated metabolism  will  be  as  follows: 

100  grams  of  protein  at  4.1  calories  =    410  Calories. 
223.6  grams  of  fat  at  9.3  ''       =  2079        " 

Total 2489        " 

This  bit  of  reckoning  is  an  instance  of  indirect  calorimetry. 
It  is  found  in  practice  that  direct  and  indirect  calorimetry 
lead  to  results  so  nearly  identical  as  to  make  it  certain  that 
the  discrepancy  between  them  is  accidental.  The  two 
figures  differ  in  modern  work  by  less  than  1  per  cent. 

The  correspondence  between  the  observed  and  the  cal- 
culated calories  means  this:  that  the  animal  displays  no 
powers  of  movement  or  heat  production  which  cannot  be 
referred  to  the  oxidation  of  organic  compounds  in  its  body, 
and  so  eventually  to  the  stores  of  chemical  energy  in  its  food. 
It  is  an  engine,  a  transformer,  and  not  a  creator  of  energy. 
A  glimpse  of  this  conception  came  within  the  vision  of  the 
brilliant  Count  Rumford  more  than  one  hundred  years  ago. 
He  had  become  convinced  that  heat  must  be  a  form  of 
energy  rather  than  a  substance.  His  conclusions  had  been 
drawn  in  part  from  the  observation  that  there  seemed  to  be 
no  limit  to  the  quantity  of  heat  obtainable  from  small 
masses  of  metal  in  the  process  of  drilling.  With  a  flash 
of  insight  he  looked  at  the  horse  which  was  walking  in  a 
circle  to  move  the  drill,  and  queried  whether  the  heat  in  the 
iron  turnings  might  not  be  a  derivative  of  the  muscular 
work  of  the  animal,  and  whether  the  same  energy  had  not 
in  some  sense  pre-existed  in  the  food  given  to  it.  The 
verification  of  his  induction  was  long  deferred,  but  has  at 
length  been  made  entirely  conclusive.  The  credit  for 
this  demonstration  is  shared  by  many  workers,  but  most 
conspicuously  by  Rubner,  in  Germany,  and  Atwater,  in 
this  country. 

Let  us  return  once  more  to  our  quantitative  illustration. 
If  the  day  of  the  experiment  had  been  one  of  ample  feeding 
instead  of  fasting,  and  the  diet  had  consisted  of  protein 
and  carbohydrate,  the  metabolism  might  have  been  held 


THE  ENERGY  OF  THE  METABOLISM       185 

to  have  involved  these  materials  rather  than  protein  and 

fat.     The  supposition  has  already  been  entertained  and 

may  now  be  made  a  basis  for  indirect  calorimetry.     From 

p.  180  we  copy  the  amounts  of  protein  and  carbohydrate, 

multiplying  by  4.1,  the  value  common  to  both: 

100  grams  protein 410  Calories. 

430      "      carbohydrate 1763 

Total 2173 

Comparing  this  sum  with  the  2489  Cal.  previously  de- 
termined, we  see  that  for  a  given  production  of  carbon  di- 
oxid  the  evolution  of  heat  will  be  greatest  when  fat  is  the 
exclusive  non-protein  source,  and  least  when  the  metabo- 
lism is  as  nearly  as  possible  on  a  carbohydrate  footing. 
The  difference  is  some  15  per  cent. 

If  both  carbohydrate  and  fat  had  participated  in  the 
decomposition  underlying  the  observed  excretion,  the  heat 
of  the  metabolism  might  have  had  any  value  intermediate 
between  2173  and  2489  Cal.  The  figure  obtained  by 
direct  calorimetry  might  in  such  a  case  of  mixed  metabolism 
be  used  to  determine  the  part  borne  by  carbohydrate  and 
fat  respectively.  How  this  could  be  done,  roughly  at  least, 
may  be  shown  here.  Suppose  the  total  heat  production  to 
be  2300  Cal.  (The  data  regarding  excreta  are  still  as- 
sumed to  be  as  before,  16  grams  of  nitrogen  and  225  grams 
of  carbon.)  We  can  first  deduct  from  2300  the  410  Cal. 
necessarily  assigned  to  the  protein  metabolism.  The 
remainder,  1890  Cal.,  stands  for  the  heat  released  in  the 
oxidation  of  glycogen  and  fat.  We  can  now  write  simul- 
taneous equations  as  below: 

Let  X  =  the  carbon  from  fat. 

y  ^=        "  "  carbohydrate. 

Then  x  -\-  y  ^^  172  (carbon  from  non-protem  sources,  page  171). 
Also  1.3  X  9.3x  +  2.5y  X  4.1  =  1890  (remembering  that  by  mul- 
tiplying the  carbon  in 
fat  by  1.3  we  obtain  the 
fat  represented,  and  that 
by  multiplying  this  re- 
sult in  turn  by  9.3  we 
get  the  calories.  The 
case  of  the  carbohydrate 
is  parallel). 


186  NUTRITIONAL   PHYSIOLOGY 

Solving  the  equations  we  obtain  the  following  (decimals 
disregarded) : 

x  =    69      90  grams  of  fat,  heat  value,    837  Calories. 

y  =  103     227        "        carbohydrate,        "       "     1054        " 

1891       " 

Thus  we  see  that  when  the  character  of  the  metabolism  is 
known  we  can  quite  accurately  predict  the  associated  heat 
production  (indirect  calorimetry),  and  when  the  heat  pro- 
duction and  the  excreta  are  known  we  can  deduce  equa- 
tions to  find  the  relative  shares  of  fat  and  carbohydrate 
in  the  general  decomposition. 


CHAPTER  XX 
THE  FACTORS  WHICH   MODIFY  METABOLISM 

The  circumstances  which  radically  affect  the  quantity 
and  kind  of  decomposition  going  on  in  the  body  must  al- 
ready be  evident.  There  can  be  no  hesitation  in  placing 
first  among  these  conditions  muscular  activity.  Minimal 
metabolism  attends  the  most  nearly  complete  state  of 
rest  which  can  be  secured.  It  is  somewhat  lower  during 
sleep  than  during  a  like  period  of  lying  awake,  doubtless 
because  the  waking  subject  cannot  so  successfully  abolish 
muscular  contraction.  When  one  is  sleeping  metabolism 
obviously  continues  in  certain  of  the  breathing  muscles 
and  the  beating  heart.  We  cannot  doubt  that  it  proceeds 
also  in  the  glands,  the  muscular  coats  of  the  alimentary 
canal,  the  nerve-centers,  and  many  other  localities.  The 
lowest  rate  at  which  the  metabolism  of  a  healthy  adult  is 
likely  to  go  on  during  sleep  is  in  the  vicinity  of  50  Cal.  per 
hour. 

Very  moderate  activity  suffices  to  double  this  rate  of  heat 
production.  More  vigorous  exercise  raises  the  figure  in 
many  cases  to  200  or  300  Cal.  per  hour,  while  we  have  records 
of  as  much  as  600  Cal.  per  hour.  This  strikingly  high  level 
was  reached  by  a  professional  bicycle  rider  exerting  himself 
to  the  utmost  upon  a  stationary  bicycle  inside  a  calor- 
imeter. The  total  for  a  day  observed  upon  an  athlete 
working  to  the  limit  of  endurance  may  reach  9000  Cal, 
Changes  so  extreme  as  these  could  hardly  fail  to  attract 
early  attention.  Lavoisier  in  the  eighteenth  century 
noticed  that  exercise  led  to  increased  consumption  of  oxy- 
gen. Everyday  experience  of  the  heating  and  the  ''wind- 
ing" effect  of  activity  would  lead  to  the  clear  impression 
of  heightened  metabolism. 

187 


188  NUTRITIONAL   PHYSIOLOGY 

When  it  had  become  plain  that  muscular  contractions 
must  involve  increased  destruction  of  material,  the  ques- 
tion arose  as  to  the  nature  of  the  substance  sacrificed. 
Chemical  teaching  in  the  middle  of  the  nineteenth  century 
was  dominated  by  the  influence  of  Liebig.  It  was  his  view 
that  the  performance  of  muscular  work  could  be  supported 
only  by  the  consumption  of  protein.  The  impression 
was  a  natural  one,  since  muscle  is  so  largely  composed  of 
protein.  He  recognized  that  carbohj^drates  and  fats  were 
oxidized  in  the  respiratory  process,  but  held  that  heat 
alone  and  not  movement  resulted  from  their  metabolism. 
The  distinction  which  Liebig  attempted  to  draw  between 
the  service  of  protein  and  non-protein  material  was  des- 
tined to  be  wiped  out  in  consequence  of  a  certain  memorable 
trial.  The  reference  is  to  the  so-called  "Faulhom  experi- 
ment" of  Fick  and  Wislicenus. 

By  the  year  1860  the  conception  of  the  convertibility 
of  energy  from  one  form  to  another  had  become  familiar. 
A  valuable  datum,  the  mechanical  equivalent  of  heat,  had 
become  available.  (One  calorie  is  equal  to  the  energy 
consumed  in  raising  426.5  kilograms  1  meter.)  The  fuel 
values  of  a  number  of  substances  were  known.  Two  young 
scientists,  Fick  and  Wislicenus,  conceived  a  project  for 
testing  the  prevailing  belief  in  the  unique  service  of  pro- 
tein as  the  support  of  muscular  acti\'ity.  They  knew  that 
the  nitrogen  of  the  urine  would  afford  the  approximate 
measure  of  the  protein  undergoing  destruction  in  a  given 
interval.  If  a  known  amount  of  work  were  done  it  would 
be  possible  to  find  out  whether  the  protein  consumed  in 
the  same  time  would  account  for  all  of  it. 

They  ascended  the  Faulhorn,  a  mountain  rising  1956 
meters  above  the  lake  at  its  foot.  In  reaching  the  top 
each  experimenter  must  have  done  an  amount  of  work 
represented  by  the  product  of  his  weight  by  the  vertical 
height  attained.  (Much  additional  work  must  have  been 
done  also — by  the  heart,  the  breathing  muscles,  and  in  the 
execution  of  other  movements — for  which  no  credit  is 
given  in  the  calculation.)     The  figured  work,  therefore, 


THE    FACTORS    WHICH    MODIFY    THE    METABOLISM      189 

was  a  minimum  quantity.  For  Fick  it  was  129,096  kilo- 
grammeters;  for  Wislicenus,  a  heavier  man,  it  was  148,656. 
The  investigators  collected  their  urine  while  on  the  path 
and  for  some  hours  after  completing  the  ascent.  They  ate 
only  non-nitrogenous  food,  that  the  excretion  might  not  be 
increased  merely  as  a  result  of  diet.  The  nitrogen  found 
in  the  urine  indicated  in  each  case  the  destruction  of  be- 
tween 30  and  40  grams  of  protein.  The  highest  theoretic 
contribution  which  such  a  quantity  of  protein  could 
have  made  to  the  work  done  would  have  been  only  half 
the  known  total,  and,  of  course,  a  smaller  part  of  the  actual 
performance.  The  inference  could  not  be  escaped;  some- 
thing other  than  protein  had  been  used  as  a  source  of 
muscular  energy. 

The  figures  obtained  by  Fick  and  Wislicenus  have  been 
revised  and  corrected  by  critics  in  view  of  later  discoveries, 
but  their  chief  significance  has  never  been  modified.  At 
the  time  of  this  celebrated  experiment  the  respiration  ap- 
paratus at  Munich  had  not  been  built.  As  soon  as  this 
plant  was  in  operation  under  the  direction  of  Voit  and 
Pettenkofer  "it  became  possible  to  learn  much  more  about 
the  effects  of  exercise  upon  metabolism.  It  was  soon  es- 
tablished that  instead  of  being  the  sole  fuel  employed  to 
furnish  muscular  energy,  protein  is  not  even  the  principal 
one.  The  nitrogen  excretion  of  a  man  on  a  mixed  diet 
does  not  increase  notably  in  a  period  of  work  as  compared 
with  what  it  is  during  rest.  On  the  other  hand,  his  out- 
put of  carbon  dioxid,  like  the  calories,  is  a  reliable  index  of 
the  degree  of  his  activity.  The  contracting  muscles  evi- 
dently employ  non-nitrogenous  material  for  oxidation  when 
the  usual  conditions  of  supply  are  maintained. 

The  question  which  naturally  follows  is  as  to  whether 
carbohydrate  or  fat  is  the  preferred  fuel.  This  cannot  be 
discussed  at  length,  but  it  seems  fair  to  say  that  both  may 
be  used  to  good  purpose  and  with  nearly  equal  economy. 
The  herbivorous  animals  may  be  assumed  to  work  most  of 
the  time  at  the  expense  of  carbohydrate  oxidized.  The 
carnivora  use  much  more  fat,  though  it  is  to  be  borne  in 


190  NUTRITIONAL   PHYSIOLOGY 

mind  that  the  large  quantities  of  protein  which  they  con- 
sume are  a  source  of  sugar  within  the  body.  Gram  for 
gram,  it  must  be  remembered,  sugar  and  fat  are  not  equiv- 
alent. One  gram  of  fat  has  the  energy  of  approximately 
2i  grams  of  carbohydrate.  Quantities  of  the  two  food- 
stuffs which  are  in  this  proportion  (and  hence  equal  in 
fuel  value)  are  said  to  be  isodynamic.  The  principle  may 
be  illustrated  as  follows :  50  grams  of  fat  is  withdrawn  from 
a  day's  ration  and  113  grams  of  starch  substituted.  A 
simple  computation  will  show  that  the  heat  value  of  the 
diet  has  neither  been  increased  nor  diminished.  The 
calories  lost  through  the  removal  of  the  fat  are  465,  while 
those  introduced  with  the  starch  are  practically  the  same. 
Of  course,  the  possibility  of  such  substitutions  is  much 
limited  by  considerations  of  palatability  and  individual 
variations  of  digestive  power. 

The  fact  that  carbohydrate  and  fat  are  freely  used  to 
yield  energy  for  muscular  movement  does  not  exclude  pro- 
tein from  such  service.  A  carnivorous  animal  can  be  kept 
for  a  long  time  strong  and  active  upon  a  diet  composed 
almost  wholly  of  protein.  Yet  it  remains  possible  that 
carbohydrate  is  the  chosen  fuel  during  feeding  of  this 
kind.  When  enough  protein  is  given  to  secure  nitrogen 
equilibrium  and  to  furnish  energy  to  the  full  extent  needed 
for  the  work  done,  the  uncombined  amino-acids  must  give 
rise  to  abundant  sugar,  and  it  may  well  be  that  this  is  the 
chief  factor  in  the  muscular  metabolism.  An  engine  is 
made  of  iron  and  steel,  but  its  regular  fuel  is  coal;  a  muscle 
is  made  essentially  of  protein,  but  protein  is  not  its  usual 
fuel.  Protein  food  must  be  needed  in  definite  amounts  for 
the  original  development  of  muscles  and  also  for  the  in- 
crease in  their  substance  under  conditions  of  training.  It 
is  not  clear  that  any  considerable  supply  is  needed  for  their 
best  working  when  the  desired  level  of  development  has 
been  reached.  This  distinction  between  growth  and 
operation  has  important  bearings  upon  theories  of  hygiene. 

Mental  States  and  Metabolism. — After  hearing  of  the 
striking  correspondence  between  the  degree  of  muscular 


THE    FACTORS    WHICH    MODIFY   THE    METABOLISM      191 

work  and  the  extent  of  metabolism,  one  is  disposed  to  ask 
what  are  the  facts  regarding  mental  work.  When  it  is 
said  that  mental  states  have  no  distinct  influence,  apart 
from  that  which  is  secondary  to  the  changes  in  tissue 
activity  which  may  attend  them,  a  feeling  akin  to  disap- 
pointment is  often  manifested.  Yet  a  little  reflection  will 
convince  one  that  positive  effects  are  scarcely  to  be  ex- 
pected. The  central  nervous  system,  unportant  as  it  is, 
constitutes,  after  all,  only  3  or  4  per  cent,  of  the  body. 
Most  of  its  substance  is  stable  and  relatively  passive  in  its 
nature.  If  its  really  active  cells  were  to  double  their 
average  metabolism  the  addition  to  the  heat  production 
and  carbon  dioxid  elmiination  of  the  subject  would  not 
be  significant  in  the  totals  observed.  A  slight  change  in 
the  state  of  the  muscles  would  suffice  to  offset  and  dis- 
guise it. 

An  emotional  experience  is  much  more  than  a  cerebral 
phenomenon.  In  times  of  excitement  the  skeletal  muscles 
are  played  upon  by  the  nervous  system  and  metabolism 
in  consequence  may  be  largely  increased.  But  this  is  not 
the  direct  result  of  the  brain  process;  it  is  merely  a  special 
case  of  muscular  activitj^  An  emotion,  pleasurable  or  the 
reverse,  is  a  kind  of  exercise  and  often  one  of  marked  inten- 
sity. This  is  recognizable  in  the  increased  heart  action, 
in  the  quickened  breathing,  and  in  the  contractions  which 
bring  about  characteristic  attitudes  and  expressions. 

Alental  application  of  a  kind  which  can  be  more  or  less 
successfully  separated  from  these  muscular  accompani- 
ments cannot  be  showTi  positively  to  affect  the  metabolism. 
A  short  time  ago  a  trial  was  made  at  Middletown,  Con- 
necticut, in  which  a  group  of  students  in  Wesleyan  Uni- 
versity took  hona-fide  examinations  in  a  respiration  cham- 
ber which  was  at  the  same  time  a  calorimeter.  Each  man 
took  his  turn,  spending  three  hours  over  his  paper  and 
experiencing  the  usual  anxiety  and  strain  attendant  on  such 
a  proceeding.  On  other  days  each  subject  spent  a  like 
period  in  the  chamber  engaged  in  copying  printed  matter. 
Thus  it  was  possible  to  compare  in  about  twenty  cases  the 


192  NUTRITIONAL   PHYSIOLOGY 

metabolism  of  a  period  of  brain  work  with  the  metabolism 
during  a  time  in  which  similar  muscular  movements  were 
made,  but  in  the  absence  of  conscious  effort.  There  was  no 
distinct  difference. 

Brain  cells  undoubtedly  have  peculiar  metabolic  prod- 
ucts and  make  demands  upon  the  blood  for  supplies  of  a 
somewhat  different  order  from  those  required  by  any 
other  tissues.  But  their  distinguishing  wastes  can  hardly 
be  recognized  when  mingled  with  the  outgo  from  so  many 
other  organs,  and  it  is  equally  difficult  to  discover  just 
what  they  appropriate  for  their  nutrition.  The  notion 
that  certain  articles  of  diet  are  brain  "foods"  rests  on  very 
unsafe  assumptions.  The  popular  association  of  phospho- 
rus with  the  brain  and  its  activity  has  no  more  justifica- 
tion than  could  be  claimed  for  sulphur  or  any  element 
present  in  the  proteins. 

From  what  has  been  said  it  will  be  evident  that  the  diet 
must  be  increased  for  the  support  of  muscular  work,  but 
that  no  more  food  is  needed  for  the  student  occupied  with 
his  books  than  for  the  same  man  at  leisure.  The  chances 
are  that  his  leisure  days  will  be  spent  in  less  sedentary 
fashion  than  his  days  of  application,  and  that  his  appetite 
will  lead  to  a  larger  consumption  in  his  so-called  resting 
time. 

Feeding  and  Metabolism. — What  can  be  said  at  this 
time  about  the  influence  of  food  upon  metabolism  must 
be  in  the  nature  of  a  summary  of  points  already  made. 
Any  effect  which  food  may  have  is  generally  so  much  less 
in  degree  than  the  effect  of  activity  as  to  be  readily  con- 
cealed by  changes  in  the  exercise  taken.  Thus,  while  it  is 
broadly  correct  to  say  that  a  fasting  man  metabohzes  less 
material  than  a  man  who  has  abundant  food,  the  relation 
will  be  reversed  if  the  starving  subject  is  compelled  to 
work  more  actively  than  his  fellow.  An  animal  which  is 
denied  food  is  economical  in  its  metabolism,  but  the  econ- 
omy is  secured  chiefly  through  its  marked  tendency  to  be 
quiet.  When  the  influence  of  muscular  activity  is  elimi- 
nated as  far  as  possible  we  can  discover  some  suggestive 


THE    FACTORS    WHICH    MODIFY   THE    METABOLISM      193 

facts  with  reference  to  the  varying  properties  of  the  differ- 
ent food-stuffs. 

Suppose  that  a  man  remains  for  two  days  in  a  calori- 
meter, the  first  being  a  day  of  fasting  and  the  second  a  day 
of  feeding.  His  occupation  on  the  two  days  is  made  as 
nearly  identical  as  may  be.  We  will  assume  that  the 
metabolism  of  the  first  day  is  found  to  be  2000  Calories. 
The  food  value  of  the  ration  allowed  for  the  second  day  may 
be  also  2000  Calories.  It  can  be  predicted  that  the  metabo- 
lism will  rise  somewhat  in  consequence  of  the  taking  of 
food,  but  the  increase  will  not  be  striking.  The  new  total 
will  perhaps  be  something  like  2300  Calories.  A  far  greater 
increment  would  have  resulted  from  the  prescribing  of 
moderate  muscular  work.  The  precise  extent  of  the  ad- 
vance will  be  conditioned  largely  upon  the  make-up  of  the 
diet.  If  protein  is  given  quite  freely  the  stimulation  of 
the  metabolism  will  be  decidedly  more  evident  than  if  the 
food  furnished  is  almost  wholly  non-nitrogenous.  Protein 
is  said  to  exert  a  specific  dynaviic  effect  upon  the  decom- 
position processes  of  the  body. 

The  divergence  between  protein  and  other  types  of  food 
in  the  matter  of  increasing  the  metabolism  has  been  care- 
fully estimated  by  Rubner.  He  has  given  us  some  helpful 
figures.  If  the  fasting  heat  production  of  an  animal  is 
represented  by  the  number  100,  the  requirement  will  not 
be  exactly  met  by  a  supply  of  any  food  having  this  value, 
but  must  be  met  by  larger  supplies.  Carbohydrate  is  the 
most  economical  of  the  three  kinds;  a  fasting  metabolism 
of  100  Calories  may  be  compensated  and  equilibrium  of 
income  and  outgo  established  by  giving  106  Calories  in  the 
form  of  starch  or  sugar.  (The  quantity  must  be  that  ac- 
tually assimilated,  not  merely  what  is  swallowed.)  In 
other  words,  feeding  an  animal  for  one  day  on  carbohydrate 
exclusively,  and  liberally  enough  to  make  the  energy  given 
equal  that  evolved,  may  be  expected  to  raise  its  meta- 
bolism some  6  per  cent,  above  the  previous  fasting  level. 

With  fat  there  is  a  slightly  more  positive  effect  in  the 
same  direction.     If  fat  is  given  to  an  animal  after  an  in- 

13 


194  NUTRITIONAL   PHYSIOLOGY 

terval  of  fasting  the  metabolism  responds  by  rising  more 
than  in  the  first  case.  Instead  of  the  6  per  cent,  increase 
about  14  per  cent,  is  to  be  anticipated.  Hence,  to  insure 
equahty  of  income  and  outgo  from  the  energy  standpoint 
114  Calories  must  be  furnished  in  the  form  of  fat  to  corre- 
spond with  a  fasting  production  of  100  Calories.  The 
specific  dynamic  effect  of  protein  is  much  more  decisive. 
If  pure  protein  is  the  only  food  the  metabolism  may  in- 
crease by  as  much  as  40  per  cent,  over  the  fasting  standard. 
The  selection  of  much  protein  at  a  meal  will  be  followed, 
during  the  next  few  hours,  by  a  definite  rise  in  heat  pro- 
duction which  may  be  a  really  wasteful  operation.  A 
quantity  of  heat  may  be  generated  entirely  apart  from  the 
performance  of  muscular  movements  and  have  to  be  re- 
moved as  a  useless  excess. 

Foods  rich  in  protein  have  the  name  of  being  "heating," 
and  here  as  in  many  another  instance  the  popular  impres- 
sion has  been  supported  by  scientific  findings.  We  can 
see  that  the  eating  of  much  meat  in  warm  weather  must 
add  to  the  discomfort  of  the  eater.  The  effect  is  some- 
thing like  the  opening  of  a  furnace  draft  when  the  house  is 
already  too  hot  for  the  pleasure  of  its  tenant.  A  distinc- 
tion must  be  made  between  the  heating  influence  of  protein 
and  the  high  fuel  value  of  fat.  It  is  true  that  fat,  gram 
for  gram,  can  liberate  more  energy  in  the  body  than  can 
protein.  But  it  is  less  likely  to  be  destroyed  when  there  is 
no  useful  application  to  be  made  of  its  energy  content. 

Age  and  Sex  as  Influencing  MetaboUsm.^ — ^ Young  and 
growing  individuals  whether  human  or  otherwise  have 
relatively  more  heat  production  than  is  the  case  with 
adults.  Thus  an  infant  weighing  10  kilograms  (22  pounds) 
may  be  expected  to  have  a  metabolism  of  600  or  700  Cal- 
ories. That  is  to  say,  that  with  a  weight  only  one-sixth 
or  one-seventh  of  that  of  a  grown  man,  it  has  a  heat  pro- 
duction one-third  or  one-fourth  as  great.  It  has  been 
shown,  however,  that  if  the  metabolism  of  both  the  infant 
and  the  adult  is  calculated  for  the  surface  of  the  body  there 
is  a  reasonable  proportionality  between  them.     The  area 


THE    FACTORS    WHICH    MODIFY    THE    METABOLISM      195 

of  the  skin  of  an  average  man  is  something  less  than  2 
square  meters.  The  minimum  metaboUsm  may  be  said  to 
be  in  the  vicinity  of  1000  Calories  per  square  meter. 

The  smaller  the  body,  if  it  is  of  a  certain  standard  shape, 
the  larger  its  surface  in  proportion  to  the  weight.  This 
is  an  important  biologic  principle.  Mathematically 
stated  it  runs  as  follows:  If  two  animals  of  similar  build  are 
compared  with  reference  to  a  given  dimension,  such  as 
length,  their  weights  will  vary  as  the  cube  and  their  sur- 
faces as  the  square  of  this  measurement.  That  is,  if  one 
animal  is  twice  as  long  as  another  it  will  weigh  eight  times 
as  much  and  have  four  times  the  surface.  Since  the  body 
loses  heat  in  proportion  to  the  extension  of  its  surface  it  is 
not  strange  that  this  is  the  determining  factor  for  the 
metabolism.  It  is  surprising  nevertheless  that  animals 
as  unlike  as  man,  mouse,  and  fowl  should  evolve  heat  in 
nearly  equal  quantities  for  a  unit  of  superficial  area. 

Broadly  speaking,  it  may  be  said  that  men  have  a  greater 
metabolism  than  women.  When  the  larger  average  stature 
of  men  is  taken  into  consideration  the  difference  is  dimin- 
ished, but  does  not  wholly  disappear.  We  must  recognize 
that  when  a  man  and  a  woman  are  equal  in  weight  there 
are  still  characteristic  features  of  organization  which  modify 
the  comparison.  Of  these  the  most  evident  is  the  greater 
prominence  of  subcutaneous  adipose  tissue  in  the  female 
figure.  When  allowance  is  made  for  this  inactive  material, 
it  is  plain  that  for  equal  weights  the  woman  has  less  truly 
living  substance  to  be  the  seat  of  metabolism.  Her  skeletal 
muscles  in  particular  are  very  unlikely  to  make  as  large 
a  mass  as  those  of  a  man  who  weighs  the  same.  The 
adipose  tissue  may  have  a  secondary  effect  also.  While 
its  presence  means  that  the  metabolism  is  confined  to  a 
more  limited  system  in  the  woman  than  in  the  man,  it 
tends  at  the  same  time  to  economize  her  heat  loss  and  so  to 
lessen  the  need  of  internal  oxidation.  Such  a  conception 
will  be  more  readily  entertained  when  attention  shall  have 
been  given  to  the  subject  matter  of  the  next  chapter. 


CHAPTER  XXI 

THE  MAINTENANCE  OF  THE  BODY 
TEMPERATURE 

All  animals  are  producers  of  heat,  and  must,  therefore, 
maintain  temperatures  above  those  of  their  surroundings 
unless  the  evaporation  of  water  more  than  compensates 
for  the  heat  of  metabolism.  We  speak,  however,  of  warm- 
blooded and  cold-blooded  animals  as  though  some  were 
far  more  liberal  than  others  in  their  calorific  output. 
This  is  actually  the  case.  But  a  better  distinction  be- 
tween the  two  classes  may  be  found  in  the  fact  that  some 
animals  allow  themselves  to  be  warmed  and  cooled  readily, 
while  others  keep  near  to  a  constant  standard  of  internal 
temperature.  We  call  a  frog  a  cold-blooded  animal,  but 
what  we  really  mean  is  that  it  accepts  for  its  own  the  tem- 
perature of  its  environment.  On  a  very  hot  day  the  frog 
may  be  as  warm  as  a  man;  in  winter  the  frog  is  chilled 
through  and  through,  while  the  tissues  of  a  human  being 
everywhere  save  at  the  surface  are  kept  at  the  same  tem- 
perature as  in  summer. 

Animals  which  have  a  fixed  standard  to  which  they  ad- 
here under  all  ordinary  conditions  are  spoken  of  as  homo- 
thermous.  This  trait  is  shared  by  birds  and  mammals. 
Our  interest,  of  course,  centers  in  man's  remarkable  capac- 
ity to  keep  his  deeper  tissues  at  the  same  level  in  spite  of 
radical  external  changes  and  equally  sweeping  changes  in 
his  own  metabolism.  The  means  by  which  this  result  is 
attained  differ  somewhat  with  different  animals  of  the 
homothermous  order.  We  shall  confine  our  attention 
almost  entirely  to  the  human  problem. 

The  common  clinical  thermometer  bears  the  mark 
* 'Normal"  opposite  the  point  on  its  scale  corresponding  to 

196 


THE    MAINTENANCE    OF   THE    BODY   TEMPERATURE      197 

98.6°  F.  (In  all  the  following  discussion  the  familiar 
Fahrenheit  system  will  be  used.)  The  standard  just 
quoted  is  for  the  mouth.  The  body  temperature  is  often 
estimated  by  placing  the  thermometer  elsewhere,  as  in  the 
armpit  or  the  rectum.  The  rectal  temperature  is  the  most 
reliable,  and  is  usually  higher  by  half  a  degree,  or  a  little 
more,  than  that  of  the  mouth.  It  is  clear  that  the  tem- 
perature of  the  skin  camiot  be  a  constant  one ;  it  is  affected 
both  by  the  external  conditions  and  by  the  variations 
of  blood-flow  in  the  superficial  vessels. 

There  are  small  but  distinct  changes  of  body  tempera- 
ture during  each  day.  The  lowest  figures  are  noted  early 
in  the  morning  before  one  has  become  active  and  when  the 
sense  of  prostration  is  apt  to  be  overpowering.  A  gentle 
upgrade  is  maintained  until  the  maximum  is  reached  in  the 
late  afternoon  or  early  evening.  The  average  extent  of  the 
rise  is  about  1°  F.  It  coincides  suggestively  with  the 
temperamental  change,  which  is  to  be  observed  from  a 
prosaic  and  literal  frame  of  mind  to  a  condition  of  emotional 
instability.  Compared  with  the  morning,  the  evening  is 
a  feverish  period.  When  there  is  actual  fever  the  diurnal 
ascent  of  the  temperature  often  adds  considerably  to  the 
restlessness  and  discomfort  of  the  patient  as  night  ap- 
proaches. 

After  we  have  made  allowance  for  such  irregularities  it 
remains  true  that  the  approximate  constancy  of  the  tem- 
perature is  one  of  the  most  wonderful  facts  which  the 
physiologist  has  to  explain.  Uniformity  of  temperature 
implies  equality  of  heat  production  and  heat  loss.  Our 
task,  then,  is  to  show  how  this  equality  is  continued  in  the 
face  of  variable  factors  tending  to  disturb  it.  Artificial 
contrivances  for  keeping  constant  temperatures  in  cer- 
tain chambers  may  be  considered  with  advantage  before  we 
attempt  to  analyze  a  mechanism  so  much  more  intricate 
than  they.  There  are  two  principles  on  which  incubators 
or  thermostats  may  be  operated:  (1)  The  heat  given  to  the 
system  may  be  increased  or  diminished  to  keep  pace 
with  the  heat  lost;  (2)  a  constant  supply  of  heat  may  be 


198  NUTRITIONAL   PHYSIOLOGY 

provided  and  adjustments  made  to  favor  its  escape  or  to 
conserve  it,  according  as  the  tendency  is  toward  a  rise  or 
fall  in  temperature. 

The  first  principle  is  illustrated  by  those  thermostats  in 
which  gas  flames  are  automatically  caused  to  rise  when  the 
apparatus  begins  to  cool  off  and  are  cut  down  when  it 
begins  to  be  warmed  above  the  intended  level.  The 
second  way  of  securing  the  same  result  is  exemplified  by 
the  common  incubator  used  for  hatching  eggs,  in  which  the 
heat  is  furnished  by  a  lamp  kept  burning  at  the  same  height 
at  all  times,  while  a  ventilator,  opening  and  closing,  dissi- 
pates or  retains  the  heat  as  required.  In  the  living 
body  we  can  recognize  the  working  of  both  these  princi- 
ples, but  it  is  rather  surprising  to  find  how  much  is  ac- 
complished by  the  second  without  the  aid  of  the  first. 
In  other  words,  the  constant  internal  temperature  is 
maintained  much  of  the  time  by  the  promotion  or  the 
retarding  of  heat  loss  without  any  appeal  to  the  tissues  for 
metabolic  support. 

Our  practice  of  adapting  our  clothing  to  the  season  and 
to  in-door  and  out-door  life  is  an  extension  of  the  means 
which  the  organism  itself  employs  for  the  same  purpose. 
Extra  clothing  hinders  the  escape  of  heat  from  the  body 
and  makes  possible  the  maintenance  of  the  normal  state 
with  no  increase  of  oxidation  in  spite  of  some  degree  of 
external  cold.  A  race  of  naked  savages  must  certainly 
vary  the  amount  of  their  metabolism  much  more  positively 
from  summer  to  winter  than  we  do  with  the  habits  of  our 
civilization.  Animals  may  enjoy  the  protection  of  heavier 
coats  in  cold  weather,  and  they  show,  moreover,  an  instinct 
to  assume  those  positions  that  reduce  to  a  minimum  their 
surface  exposure. 

To  explain  in  detail  how  changing  circumstances  are 
met,  let  us  imagine  a  man  placed  successively  in  atmo- 
spheres of  different  temperatures.  We  will  begin  with  a 
room  at  68°  F.,  a  condition  which  we  regard  as  agreeable 
and  which  we  aim  to  produce  when  artificial  heating  is  in 
use.     If  our  subject  has  spent  an  hour  in  this  room  he  has 


THE    MAINTENANCE    OF   THE    BODY   TEMPERATURE      199 

very  likely  had  a  metabolism  of  100  Calories,  and  he  has  in 
the  same  time  discharged  by  radiation,  conduction,  and 
evaporation  a  similar  amount  of  heat.  His  blood  is  some 
30  degrees  warmer  than  the  air  around  him.  Let  him  now 
take  his  place  in  a  room  where  the  thermometer  stands  at 
84°  F.  Suppose  him  to  remain  for  an  hour  in  this  disagree- 
ably warm  apartment.  His  temperature  will  be  found  to 
rise  but  little,  perhaps  not  at  all.  Yet  the  change  has  re- 
duced the  difference  between  his  own  temperature  and  that 
of  his  surroundings  from  about  30  degrees  to  about  15. 
The  tendency  of  his  environment  to  withdraw  heat  from 
his  body  must  have  been  halved.  What,  then,  has 
happened?  Has  his  metabohsm  fallen  to  50  Calories  or  has 
there  been  a  readjustment  to  facilitate  the  removal  of  the 
full  100  Calories?     The  latter  is  found  to  be  the  case. 

The  withdrawal  of  heat  from  the  body  is  favored  by  two 
reflex  changes.  One  of  these  consists  in  an  increase  in  the 
amount  of  blood  flowing  through  the  skin  and  thus  exposed 
to  the  cooling  influence  of  the  outside  air.  The  other  is 
seen  in  the  breaking  out  of  perspiration.  The  evaporation 
of  the  water  thus  brought  to  the  surface  cools  the  skin  and 
the  blood  beneath  it.  The  blood  then  mingles  with  that 
which  has  been  passing  through  the  deeper  tissues  and  the 
rising  temperature  is  checked.  It  is  well  to  insist  just  here 
that  the  appearance  of  the  skin  gives  little  indication  of  the 
rate  at  which  the  sweat  is  being  secreted.  So  long  as  evap- 
oration keeps  up  with  the  arrival  of  the  water  at  the  pores 
there  will  be  no  visible  moisture.  We  notice  the  perspira- 
tion most  when  it  is  failing  to  accomplish  its  object,  that  is, 
when  it  accumulates  instead  of  being  vaporized.  The 
evaporation  of  water  from  the  respiratory  passages  is,  of 
course,  one  means  of  removing  heat,  but  with  human 
beings  this  is  not  a  quantity  which  increases  with  external 
warming.  It  does  enter  into  the  adaptive  reaction  in  the 
case  of  animals  which  pant. 

Let  us  now  transfer  our  subject  to  the  unusual  tempera- 
ture of  105°  F.  The  radiation  and  conduction  effects  will 
now  be  reversed;  the  blood  will  tend  to  be  warmed  rather 


200  NUTRITIONAL    PHYSIOLOGY 

than  cooled  as  it  approaches  the  surface.  The  metabohsm 
will  continue  unabated.  The  body  is  thus  exposed  to 
warming  from  without,  while  it  does  not  cease  to  heat  itself 
from  within.  How  can  it  escape  a  steady  rise  of  its  in- 
ternal temperature  leading  to  prostration  and  death? 
Benjamin  Franklin  answered  this  question  when  he  pointed 
out  that  the  sole  resource  of  the  organism  in  such  a  situa- 
tion must  be  its  power  to  evaporate  water.  To  dispose  of 
100  Calories  in  an  hour  by  evaporation  alone  demands  the 
secretion  of  about  200  grams  of  water  in  the  same  time,  an 
amount  which  can  readily  be  produced. 

When  the  air  is  warm  the  humidity  has  much  to  do  with 
human  power  to  endure  the  condition.  If  there  is  full 
saturation  and  a  temperature  as  high  as  that  of  the  blood 
the  heat  of  metabolism  will  be  pent  in  the  body  and  heat- 
stroke is  inevitable  if  there  is  not  a  prompt  relief;  100  Calor- 
ies added  to  the  average  human  body  in  an  hour  will  raise 
its  temperature  by  nearly  4°  F.  A  second  hour  of  such 
an  upgrade  could  hardly  be  survived.  Men  may  live  and 
work  for  several  hours  on  a  stretch  in  dry  air  with  the  tem- 
perature around  them  as  high  as  130°  F.,  but  they  cannot 
be  active  in  saturated  air  at  90°  F.  The  first  of  these 
conditions  is  realized  in  the  stoke-holds  of  ocean  steamers ; 
the  second,  in  certain  deep  mines.  Everyone  knows  that 
the  most  trying  weather  we  have  to  put  up  with  is  not 
that  which  makes  the  record  for  the  mercury,  but  those 
days  which  are  less  warm,  but  which  we  describe  as  muggy 
or  sticky.  We  are  acceding  to  a  correct  instinct  when  we 
are  relaxed  and  indolent  under  such  circumstances. 

We  may  now  return  to  the  starting-point  and  submit  our 
imaginary  victim  to  temperatures  lower  than  68°  F.  If 
he  is  taken  to  a  room  where  the  thermometer  is  at  60°  F. 
he  will  probably  feel  chilly.  We  are  affected  more  by  a 
slight  change  in  the  vicinity  of  65°  F.  than  in  any  other 
part  of  the  scale.  The  fact  is,  apparently,  that  when  the 
external  temperature  is  cut  down  from  68°  to  60°  F.  the 
skin  temperature,  on  which  our  sensation  depends,  is  re- 
duced by  a  good  deal  more  than  8  degrees.     This  is  due  to 


THE    MAINTENANCE    OF   THE    BODY    TEMPERATURE     201 

the  reduction  in  the  volume  of  blood  in  the  cutaneous 
vessels.  It  is  the  expression  of  the  endeavor  of  the  organ- 
ism to  economize  to  the  utmost  its  outgo  of  heat.  Dis- 
comfort is  permitted  to  develop  as  an  incident  of  the  adap- 
tation. The  metabolism  still  remains  about  as  it  has  been 
throughout  the  series  of  trials. 

If  the  room  temperature  is  now  lowered  decidedly  and 
no  wraps  are  provided  the  body  can  no  longer  maintain 
itself  by  mere  economy  of  heat  loss.  It  must  shift  from 
the  method  of  the  incubator  with  a  constant  flame  and 
an  adjustable  ventilator  to  the  other  form  of  regulation — 
that  of  the  thermostat  with  variable  flame.  That  is  to 
say,  the  metabolism  must  be  stimulated.  A  sign  of  this 
rallying  on  the  part  of  the  heat-producing  tissues  is  seen 
in  the  onset  of  shivering.  This  is  obviously  a  form  of 
muscular  exercise,  and  as  such  is  attended  with  increase  of 
metabolism.  When  we  resist  the  impulse  to  shiver,  as 
we  sometimes  do,  we  merely  adopt  another  kind  of  con- 
tractile activity  with  its  accompanying  contribution  of  heat 
to  the  body.  If  we  analyze  the  experience  of  being  cold 
we  find  that  we  can  recognize  a  disposition  to  muscular 
tenseness,  while  it  is  familiar  enough  that  when  the  cold 
is  severe  we  cannot  keep  from  moving  briskly  and  so  sup- 
plying the  necessary  heat.  It  is  desirable  to  reiterate  that 
the  muscles  are  not  merely  organs  of  movement,  but  our 
main  reliance  for  heat  production.  Cold  weather  generally 
means  large  metabolism,  but  the  connection  is  indirect;  the 
increase  is  only  a  special  case  of  the  rise  always  associated 
with  exercise .  So ,  too ,  the  increase  of  appetite  which  is  usual 
in  winter  is  secondary  to  the  greater  use  of  the  muscles. 

Humidity  makes  for  discomfort  in  cold  as  well  as  in 
warm  weather.  It  seems  at  first  unreasonable  to  say  that 
moisture  can  make  us  more  sensitive  to  heat  in  summer 
and  also  to  cold  in  winter,  yet  this  is  true.  The  climate  of 
our  Atlantic  coast  is  notorious  for  the  ''penetrating"  char- 
acter of  its  cold,  and  this  in  spite  of  the  fact  that  the 
thermometer  does  not  fall  so  low  as  it  does  a  short  distance 
inland.     The  paradox  is  easily  explained.     We  have  said 


202  NUTRITIONAL    PHYSIOLOGY 

that  high  humidity  in  hot  weather  interferes  with  our  com- 
fort and  efficiency  by  hindering  free  evaporation.  In 
winter  it  has  no  such  influence,  because  even  though  the 
cold  air  is  fully  saturated  it  ceases  to  be  so  when  it  has  been 
warmed  by  contact  with  the  skin.  Evaporation  can, 
therefore,  never  be  retarded  seriously  by  moisture  in  cool 
air.  What  we  notice  in  cold  weather  is  the  increased  con- 
ducting power  of  air  containing  water  vapor.  Damp  air 
may  fairly  be  said  to  partake  of  the  nature  of  the  water 
that  is  in  it;  water  feels  colder  to  the  hand  than  does  air  of 
the  same  temperature  because  it  abstracts  the  heat  more 
rapidly.  Similarly,  moist  cold  air  takes  heat  more  rap- 
idly than  does  dry  cold  air.  This  property  is  present 
just  as  surely  in  warm  humid  air,  but  it  can  affect  us  only 
when  there  is  a  wide  difference  in  temperature  between  the 
skin  and  the  surroundings. 

Temperature  Maintenance  During  Exercise. — We  have 
been  discussing  the  ways  in  which  the  human  body  guards 
itself  against  changes  of  temperature  which  tend  to  impress 
themselves  upon  it  from  the  outside.  Another  question 
is  in  regard  to  how  it  escapes  the  tendency  to  overheat 
itself  when,  during  exercise,  its  metabolism  is  doubled, 
trebled,  or  even  more  strikingly  augmented.  Reflection 
shows  that  it  employs  the  two  reflexes  on  which  it  relies 
for  defense  against  the  heat  of  the  warm  room,  namely, 
increased  surface  blood-flow  and  increased  perspiration. 
These  are  rendered  more  efficient  by  the  hastened  circula- 
tion, a  condition  not  produced  in  any  great  degree  by  ex- 
ternal heat  without  the  activity. 

A  supplementary  factor  exists  in  the  deepened  breathing 
which  takes  heat  from  the  system,  both  in  the  act  of  warm- 
ing the  respired  air  and  in  the  process  of  saturating  it. 
Still  another  factor  can  be  recognized  in  the  fanning  effect 
of  the  movements  of  parts  of  the  body  or  of  the  body  as  a 
whole.  A  man  running  brings  the  exposed  portions  of  his 
skin  constantly  into  contact  with  fresh  volumes  of  air  and 
slips  away  from  the  air  which  he  has  just  warmed  and 
saturated.  The  cooling  of  his  blood  is  in  this  way  con- 
siderably facilitated.     If  he  is  not  progressing  through 


THE    MAINTENANCE    OF   THE    BODY   TEMPERATURE     203 

space,  but  carrying  on  his  activities  in  one  place — for 
example,  in  sawing  wood — his  arms  and  trunk  still  make 
short  excursions  and  exchange  old  air  for  new.  A  breeze 
makes  a  considerable  difference  with  the  heat  output  of  a 
man's  body. 

Fever. — When  the  body  temperature  is  found  to  rise 
above  its  normal  level  and  to  persist  at  an  elevation  which 
brings  many  ill  consequences  upon  the  subject,  what  shall 
we  name  as  the  cause  of  the  disorder?  Shall  we  say  that 
the  metabolism  is  excessive?  Studies  made  upon  fever 
patients  show  that  their  nitrogenous  metabolism  is  often 
surprisingly  high,  an  indication  of  rapid  destruction  of  the 
tissues,  but  the  total  heat  production  is  not  impressively 
large.  We  shall  more  nearly  express  the  facts  if  we  say 
that  there  is  interference  with  heat  loss.  Frequently  we 
can  see  evidence  of  a  withholding  of  the  perspiration. 
Perhaps  it  is  best  to  say  that  the  central  fact  in  fever  is  the 
setting  up  of  a  false  standard  by  the  nervous  system  to 
which  for  a  time  there  is  an  adherence  as  strict  as  that 
obtaining  in  health  for  the  normal  one.  This  perverted 
action  of  the  nervous  system  is  brought  about  by  the 
poisons  in  the  circulation  at  such  times.  Transient  fever 
may  be  brought  on  by  very  severe  exercise ;  it  is  experienced 
by  men  who  run  Marathon  races.  In  these  cases  the  con- 
trolling centers  are  probably  acting  in  the  normal  way,  but 
cannot  secure  the  complete  removal  of  the  extraordinary 
quantities  of  heat  which  are  produced. 

Summary. — The  maintenance  of  a  nearly  uniform  body 
temperature  is  the  result  of  a  balance  between  heat  evolved 
and  heat  dissipated.  So  long  as  the  external  conditions 
are  not  such  as  to  cause  shivering,  muscular  tension,  or 
instinctive  activity  of  some  other  form  the  organism  regu- 
lates its  temperature  almost  wholly  by  making  adjust- 
ments to  promote  or  to  restrict  the  loss  of  heat.  The 
wearing  of  clothing  adapted  to  the  season  makes  it  possible 
to  minimize  the  demands  made  upon  the  muscles  for  extra 
heat.  Decidedly  low  temperatures  and  exceptional  ex- 
posure can  be  withstood  only  by  calling  upon  the  contrac- 
tile tissues  for  an  increased  heat  production. 


CHAPTER  XXII 
THE  HYGIENE   OF  NUTRITION 

It  is  convenient  to  make  a  division  under  this  general 
heading  between  those  factors  not  directly  connected  with 
the  diet,  which  none  the  less  deserve  consideration,  and 
those  which  do  concern  the  choice  of  food.  Probably  too 
little  attention  is  paid  to  the  fact  that  disorders  of  digestion 
and  nutrition  frequently  arise  when  the  food  eaten  is  above 
criticism,  both  as  to  quantity  and  kind. 

Nervous  Conditions  Affecting  Digestion. — Enough  has 
already  been  said  to  make  it  plain  that  the  processes  oc- 
curring in  the  alimentary  canal  are  greatly  subject  to 
influences  radiating  from  the  brain.  It  is  especially  strik- 
ing that  both  the  movements  of  the  stomach  and  the  secre- 
tion of  the  gastric  juice  may  be  inhibited  as  a  result  of 
disturbing  circumstances.  Intestinal  movements  may  be 
modified  in  similar  fashion.  Emphasis  has  been  placed  on 
the  dependence  of  the  whole  digestive  process  upon  a  good 
start.  This  can  hardly  be  too  strongly  enforced.  The 
good  start  can  scarcely  be  secured  if  the  mental  state  of  the 
subject  is  not  favorable  to  the  enjoyment  of  his  meal. 

Cannon  has  collected  various  instances  of  the  suspension 
of  digestion  in  consequence  of  disagreeable  experiences, 
and  it  would  be  easy  for  almost  anyone  to  add  to  his  list. 
He  tells  us,  for  example,  of  the  case  of  a  woman  whose 
stomach  was  emptied  under  the  direction  of  a  specialist 
in  order  to  ascertain  the  degree  of  digestion  undergone  by  a 
prescribed  breakfast.  The  dinner  of  the  night  before  was 
recovered  and  was  found  almost  unaltered.  Inquiry  led 
to  the  discovery  that  the  woman  had  passed  a  night  of 
intense  agitation  as  the  result  of  misconduct  on  the  part  of 

204 


THE    HYGIENE   OF    NUTRITION  205 

her  husband.  People  who  are  seasick  some  hours  after  a 
meal  often  vomit  undigested  food.  Apprehension  of 
being  sick  has  probably  inhibited  the  gastric  activities. 

Just  as  a  single  occasion  of  painful  emotion  may  lead  to 
a  passing  digestive  disturbance,  so  continued .  mental  de- 
pression, worry,  or  grief  may  permanently  impair  the 
working  of  the  tract  and  undermine  the  vigor  and  capacity 
of  the  sufferer.  Homesickness  is  not  to  be  regarded  lightly 
as  a  cause  of  malnutrition.  Companionship  is  a  powerful 
promoter  of  assimilation.  The  attractive  serving  of  food, 
a  pleasant  room,  and  good  ventilation  are  of  high  import- 
ance. The  lack  of  all  these,  so  commonly  faced  by  the 
lonely  student  or  the  young  man  making  a  start  in  a  strange 
city,  may  be  to  some  extent  counteracted  by  the  cultiva- 
tion of  optimism  and  the  mental  discipline  which  makes  it 
possible  to  detach  one's  self  from  sordid  surroundings. 
Alcohol  works  to  the  same  end,  but  is  a  perilous  resource 
under  these  circumstances. 

Children  are  very  often  the  victims  of  sharp  attacks  of 
indigestion.  Their  sicknesses,  which  are  accepted  with 
little  surprise  in  many  families,  are  almost  always  held  to  be 
due  to  injudicious  eating.  While  this  is  a  reasonable  belief 
in  many  cases,  it  may  be  asked  whether  emotional  causes 
of  indigestion  in  children  are  considered  as  much  as  they 
should  be.  How  common  it  is  to  see  children  made  to  cry 
while  at  the  table  by  ill-timed  rebukes.  The  quick  temper 
and  thoughtlessness  of  parents  destroys  the  happiness  of 
many  a  meal.  Granting  that  instruction  in  table-manners 
ought  to  be  given  at  suitable  times,  one  may  still  protest 
against  the  downright  rudeness  of  elders  toward  children 
when  it  is  shown  in  the  infliction  of  ridicule  and  humilia- 
tion. Consideration  of  others'  feelings  is  the  finest  element 
in  deportment. 

Moreover,  it  is  probably  fair  to  claim  that  children  may 
be  injured  by  being  forced  to  eat  food  which  they  dislike. 
The  aversions  of  early  life  are  singularly  strong.  What 
passes  for  a  foolish  whim  may  be  an  instinctive  loathing. 
There  is  an  element  of  hypocrisy  in  the  attitude  of  parents 


206  NUTRITIONAL   PHYSIOLOGY 

who  are  selecting  precisely  what  they  please  to  eat,  while 
compeUing  little  children  to  swallow  food  which  repels. 
To  obUge  a  child  to  finish  a  plateful  of  food  against  its 
inclination  may  be  crass  brutahty.  Of  course,  children 
cannot  be  humored  in  the  selection  of  eccentric  diets,  but- 
they  need  not  be  made  to  eat  when  they  would  rather  go 
hungry.  There  is  little  likelihood  that  they  will  refuse  the 
staple  articles. 

Unhappiness  may  give  rise  to  digestive  difficulties  which 
do  not  disappear  with  the  removal  of  the  first  cause.  It 
is  not  hard  to  show  how  this  may  be.  Suppose  that  the 
power  to  digest  and  absorb  food  is  lessened  by  central  in- 
hibitions. A  consequence  is  likely  to  be  the  accumulation 
of  unabsorbed  organic  material  in  the  colon  and  perhaps 
higher  up  as  well.  Bacterial  decomposition  will  bq  fos- 
tered. Some  of  the  products  of  such  a  process  may  be 
sufficiently  like  the  normal  products  of  enzyme  action  to 
play  a  part  in  nutrition,  but  others  will  probably  prove 
distinctly  detrimental.  With  the  entrance  into  the  circu- 
lation of  such  bodies  there  is  originated  what  is  known  as 
auto-intoxication. 

Long  ago  it  was  recognized  that  the  reception  into  the 
system  of  bacterial  products  might  be  a  cause  of  general 
ill  health,  of  headache,  and  of  somnolence.  Within  a  few 
years  the  impression  has  gained  ground  that  poisons  from 
the  colon  have  a  much  larger  and  more  definite  share  in  the 
development  of  disease.  Much  that  goes  by  the  name  of 
rheumatism  appears  traceable  to  this  source.  Some  of  the 
toxic  compounds  seem  to  have  the  property  of  dissolving 
the  red  corpuscles  of  the  blood,  leading  thus  to  anemia  and 
the  serious  crippling  of  the  energies  which  accompanies  it. 
Nervous  symptoms  are  among  the  most  frequent  nowadays 
referred  to  this  condition,  seemingly  so  remote  from  the 
brain. 

Physicians  apply  the  term  'Vicious  cycle"  to  a  set  of 
conditions  in  which  the  establishment  of  one  tends  to  ac- 
centuate the  others,  and  these,  in  their  turn,  add  to  the 
intensity  of  the  first.     We  can  see  how  a  vicious  cycle  may 


THE    HYGIENE    OF   NUTRITION  207 

become  operative  in  the  case  of  a  person  whose  digestive 
abihties  have  been  once  reduced  by  mental  depression. 
Auto-intoxication  may  be  induced  and  one  of  the  clearest 
results  is  a  further  depression  of  spirits.  In  this  way  a 
lasting  injury  may  be  done  and  the  indigestion  which  be- 
gan as  the  effect  of  temporary  unhappiness  may  be  per- 
petuated in  spite  of  the  return  of  favorable  circumstances. 
Auto-intoxication  may  come  from  errors  of  diet  as  well  as 
from  emotional  causes,  and  a  further  discussion  of  it  will 
be  postponed. 

Physical  fatigue  as  well  as  mental  may  interfere  with  the 
progress  of  digestion.  It  is  well  that  the  appetite  usually 
flags  in  times  of  exhaustion,  so  that  one  is  in  a  measure 
insured  against  the  tendency  to  overtax  weakened  organs. 
But  when  the  fatigue  is  habitual  there  is  an  unfortunate 
dilemma;  the  body  must  have  abundant  food  to  support 
the  heavy  labor  and  it  is  not  well  able  to  care  for  the  food 
eaten.  Loss  of  weight  is  common  when  a  man  is  so  situ- 
ated. Deprivation  of  sleep  emphasizes  this  state  of  affairs, 
and  by  dulling  both  the  appetite  and  the  digestive  capacity 
it  proves  for  many  people  the  surest  means  of  reducing 
adipose  tissue. 

Other  possible  causes  of  indigestion  may  be  mentioned 
briefly.  Taking  cold  is  one  of  these.  While  the  congestion 
and  inflammation  which  so  often  follow  exposure  to  drafts 
and  dampness  are  most  frequently  centered  in  the  mucous 
membranes  of  the  nose  and  throat,  a  corresponding  involve- 
ment of  the  alimentary  canal  is  not  rare.  Diarrheal  at- 
tacks are  common  in  the  spring  and  fall  when  the  weather 
changes  are  erratic.  They  are  probably  to  be  classed  as 
intestinal  "colds."  Another  source  of  alimentary  trouble 
is  to  be  found  in  uncorrected  defects  of  vision.  While 
headaches  are  the  most  persistent  symptoms  of  astig- 
matism and  other  ocular  imperfections,  indigestion  is  not 
uncommon,  and  its  disappearance  when  glasses  are  worn 
seems  sometimes  almost  magical. 

Quantity  of  Food. — Is  overeating  the  prevalent  error 
of  the  race?     Is  undereating  easily  possible?     These  are 


208  NUTRITIONAL   PHYSIOLOGY 

questions  which  call  for  open-minded  treatment  and  re- 
garding which  it  is  hard  to  exclude  personal  bias.  A 
writer's  love  for  the  pleasures  of  the  table,  or  his  ascetic 
superiority  to  them,  must  tincture  his  expressed  views. 
The  present  author  once  committed  himself  to  the  state- 
ment that  overeating  is  the  rule  with  men  and  the  excep- 
tion with  women.  A  pupil — a  girl — rendered  the  teaching 
in  her  examination  as  follows:  ''Women  rarely  overeat, 
men  do  constantly,  and  are,  as  a  result,  bulky  and  stupid." 
The  assertion,  though  radical,  is  exceedingly  well  worth 
considering. 

The  claim  is  often  made  that  the  average  practice  of 
mankind  must  be  the  expression  of  a  correct  biologic  in- 
stinct. Against  this  it  is  urged  that  our  own  generation 
may  inherit  appetites  which  were  adapted  to  spur  our 
ancestors  to  find  food  when  the  quest  was  difficult.  Such 
appetites  may  be  false  guides  v/hen  no  effort  is  required 
to  obtain  the  means  of  satisfaction.  Variety  of  food 
may  lead  to  overconsumption.  Modern  conditions  make 
it  possible  to  have  many  kinds  of  food  and  interesting 
contrasts  of  flavor  which  encourage  eating  for  sensuous 
gratification.  The  primitive  diet  was  monotonous  and 
unseasoned. 

Economic  factors  are  tending  to  lessen  the  individual 
ration,  and  a  great  mass  of  published  instruction  influences 
people  in  the  same  direction.  It  is  probable  that  the 
American  breakfast  is  a  much  less  substantial  meal  to- 
day than  it  was  twenty  years  ago.  The  use  of  fruit  has  in- 
creased and  food  of  greater  fuel  value  has  been  displaced. 
Odd  patent  cereals,  small  quantities  of  which  exhaust  the 
appetite,  have  been  widely  substituted  for  the  reliable  oat- 
meal. Meat  and  potato  are  not  demanded.  Lighter 
lunches  at  noon  are  the  rule,  and  though  the  late  dinner 
may  be  a  heavier  meal  than  the  old-time  supper,  the  day's 
ration  seems  to  have  definitely  diminished.  This  is  par- 
ticularly true  of  protein.  The  choice  of  100  grams  of 
nitrogenous  food,  an  amount  once  described  as  average,  is 
now  found  to  be  exceptional.     If  any  one  of  the  three  main 


THE   HYGIENE    OF    NUTRITION  209 

classes  of  food-stuffs  is  used  more  freely  than  formerly, 
it  is  the  carbohydrate  group. 

Protests  against  overeating  and  the  advocacy  of  few 
and  simple  foods  have  been  heard  from  time  to  time  all 
along  the  course  of  history.  More  than  one  recent  com- 
mentator on  these  movements  has  cited  the  story  of  the 
captive  Israelites  in  Babylon.  The  four  young  men  who 
excelled  all  other  members  of  the  king's  household  were 
those  who  had  substituted  a  diet  of  cereals  for  the  meat  and 
wine  that  were  urged  upon  them.  They  were  the  proto- 
types of  Sylvester  Graham  and  Horace  Fletcher.  The 
general  teaching  that  less  food  should  be  eaten  is  very 
often  coupled  wdth  an  approach  to  vegetarianism,  if  not 
its  downright  adoption. 

An  earlier  reference  has  been  made  to  Graham.  He  was 
a  gifted  and  well-educated  New  Englander,  born  in  1794 
and  dying  in  1851.  Fitted  for  the  ministry,  he  employed 
his  great  talents  as  a  public  speaker,  chiefly  in  the  temper- 
ance cause  and  in  support  of  his  dietetic  doctrines.  His 
''Lectures  on  the  Science  of  Human  Life,"  published  in 
1839,  constitute  an  impressive  work.  He  had  a  large 
following,  and  is  said  to  have  injured  the  trade  of  the 
butchers  in  some  localities  to  such  an  extent  that  he  was  in 
danger  of  mob  violence  at  their  hands.  Aside  from  his 
extreme  recommendation  of  the  inclusion  of  husks  and 
bran  with  food,  he  taught  practically  what  we  are  continu- 
ally hearing  at  the  present  time:  that  the  diet  should  be 
limited  in  quantity  and  variety,  that  it  should  be  unstimu- 
lating,  and  that  it  ought  to  be  eaten  "slowly  and  cheer- 
fully." 

In  our  own  day  we  have  become  used  to  the  claim  that 
most  men  would  attain  to  better  health  and  greater  effici- 
ency if  they  would  reduce  their  rations  by  25  per  cent,  or 
more.  Conditions  of  life  which  were  formerly  held  to  war- 
rant individual  allowances  of  2500  or  3000  Calories  are 
now  said  to  be  met  by  the  supply  of  2000  or  even  of  1800 
Calories.  The  most  authoritative  statements  to  this  effect 
have  come  from  the  physiologists  of  the  Sheffield  Scientific 

14 


210  NUTRITIONAL   PHYSIOLOGY 

School.  In  a  more  popular  form  the  same  ideas  have  been 
attractively  presented  by  Mr.  Fletcher. 

The  experience  and  the  principles  of  this  gentleman  are 
somewhat  familiar  to  the  American  public.  By  his  prac- 
tice of  protracted  mastication  he  contrives  to  satisfy  the 
appetite  while  taking  an  exceptionally  small  amount  of 
food.  Salivary  digestion  is  favored  and  the  mechanical 
subdivision  of  the  food  is  carried  to  an  extreme  point. 
Remarkably  complete  digestion  and  absorption  follow. 
By  faithfully  pursuing  this  system  Mr.  Fletcher  has  vastly 
bettered  his  general  health,  and  is  a  rare  example  of  muscu- 
lar and  mental  power  for  a  man  above  sixty  years  of  age. 
He  is  a  vigorous  pedestrian  and  mountain-climber  and 
holds  surprising  records  for  endurance  tests  in  the  gjma- 
nasium. 

The  chief  gain  observed  in  his  case,  as  in  others  which 
are  more  or  less  parallel,  is  the  acquiring  of  immunity  to 
fatigue,  both  muscular  and  central.  It  is  not  claimed  that 
the  sparing  diet  confers  great  strength  for  momentary 
efforts — ''explosive  strength,"  as  the  term  goes — but  that 
moderate  muscular  contractions  may  be  repeated  many 
times  with  far  less  discomfort  than  before.  The  inference 
appears  to  be  that  the  subject  who  eats  more  than  is  best 
has  in  his  circulation  and  his  tissues  by-products  which  act 
like  the  muscular  waste  which  is  normally  responsible  for 
fatigue.  According  to  this  conception  he  is  never  really 
fresh  for  his  task,  but  is  obliged  to  start  with  a  handicap. 
When  he  reduces  his  diet  the  cells  and  fluids  of  his  body 
free  themselves  of  these  by-products  and  he  realizes  a 
capacity  quite  unguessed  in  the  past. 

The  same  assumption  explains  the  fact  mentioned  by 
Mr.  Fletcher,  that  the  hours  of  sleep  can  be  reduced  de- 
cidedly when  the  diet  is  cut  down.  It  would  seem  as 
though  a  part  of  our  sleep  might  often  be  due  to  avoidable 
auto-intoxication.  If  one  can  shorten  his  nightly  sleep 
without  feeling  the  worse  for  it  this  is  an  important  gain. 
The  small  ration  decreases  the  contents  of  the  colon  in 
two  ways :  First,  the  food  residues  themselves  are  minimal, 


THE   HYGIENE   OF   NUTRITION  211 

and,  second,  the  secretions  of  the  digestive  tract  are  less 
voluminous  when  there  is  only  a  light  task  for  them  to 
perform.  Well-marked  constipation  is  established,  the 
Fletcherite  having  only  one  or  two  evacuations  in  a  week, 
but  the  material  retained  is  apparently  innocuous. 

Many  of  the  ills  referable  to  auto-intoxication  often  dis- 
appear when  the  practice  of  prolonged  chewing  is  followed. 
This  could  be  explained  as  resulting  from  the  limitation 
of  colon  accumulations,  but  seems  in  part  to  proceed  from 
the  strange  fact  that  under  the  system  the  consumption 
of  protein  invariably  shrinks  even  more  than  in  proportion 
to  the  general  restriction  of  the  food.  This  occurs  when  the 
subjects  are  guided  entirely  by  instinct  and  have  no  the- 
ories in  regard  to  the  matter.  Indifference  to  meat  or  even 
an  antipathy  to  it  may  be  developed. 

One  does  not  have  to  look  far  to  see  examples  of  the 
practical  working  of  the  Fletcher  system,  though  the  per- 
sons who  illustrate  it  may  have  no  associations  with  the 
name.  Middle-aged  and  elderly  housewives  are  to  be 
found  in  large  numbers  who  are  slow  and  frugal  eaters,  who 
care  little  for  meat,  who  are  constipated,  and  who  are  mar- 
vels of  endurance  in  the  execution  of  their  hard  tasks. 
They  sleep  lightly  and  rise  early — habits  diagnostic  of 
Fletcherism.  They  live  to  great  ages,  restlessly  active  to 
the  last.  They  show  the  strong  points  of  the  regimen;  do 
they  display  any  drawbacks  attributable  to  it? 

It  is  certainly  possible  to  undereat.  There  can  be  no 
reasonable  doubt  that  the  condition  of  the  very  poor  would 
be  bettered  if  they  could  have  more  food,  even  though  it 
were  of  no  finer  quality  than  that  to  which  they  are 
accustomed.  Crich ton-Browne,  an  English  writer,  has 
acutely  pointed  out  that  the  allowance  of  the  poorest  classes 
comes  near  to  the  ideal  of  the  New  Haven  School,  and  that 
it  is  also  almost  identical  with  the  "punishment  diet"  of 
British  prisons.  It  may  well  be  that  a  diet  which  is  suc- 
cessful in  connection  with  the  best  housing  conditions  and 
a  life  abounding  in  stimuli  to  the  intellect  and  the  feelings 
may  not  support  cheerfulness  and  vigor  in  those  whose 


212  NUTRITIONAL   PHYSIOLOGY 

surroundings  are  wretched.  Underfeeding  produces  gloom 
and  moroseness  unless  other  circumstances  are  more  in- 
fluential and  oppose  it. 

Previous  experience  is  something  to  be  considered  when 
seeking  to  judge  whether  an  individual  ought  to  reduce  his 
food-supply.  If  he  has  lived  in  luxury  and  under  every 
temptation  to  indulgence,  if  he  is  overweight  as  a  result 
of  his  habits,  short  of  breath  and  drowsy,  he  can  probably 
profit  by  the  expedient.  If  he  has  been  restrained  from 
satisfying  his  appetite  by  sheer  poverty  or  by  voluntary 
sacrifices,  if  he  is  thin,  sensitive  to  cold,  and  subject  to 
insomnia,  it  is  quite  likely  that  an  increase  of  food  will  make 
him  stronger  and  more  energetic.  Under  eating  is  often 
coupled  with  indigestion  in  such  a  way  that  it  is  impossible 
to  say  which  is  the  cause  and  which  the  effect.  Within 
wide  limits  the  digestive  glands  accommodate  themselves 
to  the  tax  levied  upon  them,  providing  adequate  amounts 
of  their  juices  for  large  or  small  meals.  Low  diet,  there- 
fore, may  weaken  the  adaptibility  of  the  organs  it  is 
designed  to  spare. 

Benedict,  of  the  Carnegie  Nutrition  Laboratory,  has 
proved  himself  an  exceedingly  shrewd  critic  of  those  writers 
who  go  to  extreme  lengths  in  their  championship  of  re- 
duced feeding.  He  has  shown  that  when  the  food  supply 
is  very  small  the  absorption  is  often  less  complete  than  with 
a  more  liberal  income,  a  clear  sign  that  the  digestive  sys- 
tem is  not  in  such  good  working  order  as  it  might  be  with 
more  to  do.  Professor  Chittenden,  the  foremost  advocate 
of  careful  restriction,  agrees  with  his  critic  to  this  extent 
at  least:  he  believes  that  an  occasional  banquet  is  a 
valuable  means  of  testing  the  canal  to  see  whether  it  has 
retained  the  reserve  power  which  it  is  desirable  to  have. 
Many  people  of  the  economical  type  characterized  just  now 
have  lost  the  faculty  of  enjoying  a  large  meal  and  cannot 
easily  increase  their  income  when  the  physician  recom- 
mends it. 

The  Peculiarities  of  Protein. — While  the  advice  to 
limit  all  forms  of  food  is  constantly  heard,  it  is  the  use  of 


THE   HYGIENE   OF  NUTRITION  213 

much  protein  which  is  most  vigorously  condemned. 
The  decisive  effect  of  a  large  protein  allowance  upon  the 
metabolism — an  effect  in  the  direction  of  a  seemingly  use- 
less increase — has  been  mentioned.  Other  peculiar  prop- 
erties call  for  discussion.  We  may  conveniently  distin- 
guish the  influences  proceeding  from  an  excess  of  protein 
in  the  intestine  from  those  which  are  associated  with  an 
excess  of  nitrogenous  metabolism.  In  regard  to  the  first 
it  may  be  said  that  protein  far  more  than  the  non- 
nitrogenous  foods  is  capable  of  generating  toxic  sub- 
stances and  so  of  becoming  a  cause  of  true  auto-intoxi- 
cation. An  unabsorbed  surplus  of  protein  is,  therefore, 
to  be  avoided. 

If,  however,  the  absorption  is  as  complete  as  can  be 
desired,  reasons  can  still  be  given  for  keeping  the  protein 
income  of  the  body  relatively  low  and  depending  largely 
upon  carbohydrates  and  fats  in  preference  to  nitrogenous 
food.  Emphasis  has  been  placed  elsewhere  on  the  sim- 
plicity of  the  normal  oxidation  products  formed  from  sugar 
and  fats  in  contrast  to  the  numerous  and  rather  complex 
bodies  which  arise  from  the  working  over  of  the  protein 
derivatives.  Carbon  dioxid  and  water  are  disposed  of  by 
the  healthy  system  with  apparent  ease.  The  nitrogenous 
wastes  require  preliminary  treatment  by  the  liver  and  per- 
haps by  other  organs,  and  must  then  be  removed  from  the 
blood  by  the  kidneys.  The  principal  one,  urea,  is  not 
commonly  a  source  of  trouble,  but  the  minor  attendants, 
such  as  uric  acid,  are  viewed  with  disfavor. 

One  is  naturally  led  to  the  opinion  that  high  protein 
feeding  must  lay  a  burden  upon  the  liver  and  the  kidneys, 
but  it  is  probably  just  to  assume  that  what  is  a  severe 
tax  for  one  person  may  not  be  so  for  another.  The  native 
efficiency  of  these  organs  is  doubtless  as  variable  as  many 
other  inherited  qualities.  Nitrogenous  by-  or  end-pro- 
ducts, whether  having  their  origin  in  the  decompositions 
within  the  colon  or  in  the  metabolism,  are  presumably 
responsible  for  the  premature  fatigue  of  which  we  have 
spoken.     An  additional  injury  which  may  be  laid  to  their 


214  NUTRITIONAL   PHYSIOLOGY 

charge  must  now  be  considered.  This  is  the  production  in 
later  Hfe  of  arteriosclerosis. 

The  saying  is  current  among  physicians  that  "a>  man  is 
as  old  as  his  arteries."  It  is  certainly  a  fact  that  the 
stiffening  of  these  vessels  is  a  chief  cause  of  malnutrition 
and  waning  power  in  the  tissues  of  the  aged.  If  arterio- 
sclerosis sets  in  prematurely,  other  features  of  a  senile 
decline  may  be  expected.  Metchnikoff  has  sought  to 
establish  a  connection  between  the  overconsumption  of 
proteins  and  early  loss  of  elasticity  and  adaptability  in  the 
human  circulatory  apparatus.  Such  a  connection  has 
long  been  granted  to  hold  for  alcohol  We  seem  brought 
to  admit  that  this  serious  impairment  of  efficiency  may 
spring  from  intemperance  in  eating  as  well  as  in  drinking. 

The  postponement  of  old  age  by  frugahty  in  feeding 
seems  in  a  measure  possible.  Yet  we  must  remember  that 
self-denial  in  this  respect  may  defeat  its  own  end,  since  it 
may  too  greatly  weaken  the  digestive  capacity.  An  al- 
ternative to  low  diet,  it  has  been  suggested,  may  be  found 
in  the  deliberate  regulation  of  the  bacterial  conditions  in 
the  intestine.  The  claim  is  made,  and  seemingly  with  good 
reason,  that  a  harmless  type  of  fermentation  may  be  en- 
couraged with  the  result  that  the  undesirable  decompo- 
sition may  be  prevented.  This  is  the  theory  underlying 
the  various  modes  of  sour-milk  treatment  so  much  in 
vogue  during  the  last  five  years.  Lactic  acid  in  moderate 
amounts  appears  to  be  quite  devoid  of  danger  to  one's 
health.  Its  presence  in  the  canal  is  hostile  to  the  develop- 
ment of  the  organisms,  which  cause  radical  putrefaction 
of  the  proteins  and  definite  auto-intoxication.  A  domi- 
nant acid  fermentation  may  be  secured  either  by  the 
taking  of  sour  milk  (kephir,  koumiss,  matzoon,  etc.)  or  by 
swallowing  from  time  to  time  cultures  of  the  lactic  organ- 
isms, together  with  sugar  for  them  to  work  on. 

The  comparative  harmlessness  of  even  extreme  constipa- 
tion when  associated  with  sparing  feeding  and  active 
absorption  has  been  granted.  Such  inaction  of  the  intes- 
tine when  it  accompanies  more  liberal  indulgence  in  food 


THE   HYGIENE   OF   NUTRITION  215 

must  be  unfortunate  and  must  favor  some  phases  of  auto- 
intoxication. Many  people  resort  periodically  to  the  use 
of  cathartics  when  they  feel  slightly  "under  the  weather," 
and  enjoy  a  buoyant  recovery  of  energy  and  ambition 
when  the  disturbance  is  over.  It  is  natural  to  interpret 
such  an  experience  as  showing,  first,  the  existence  of  a 
source  of  poisoning,  and,  second,  its  successful  removal. 
The  wise  man,  however,  will  not  be  satisfied  with  knowing 
a  way  out  of  such  disorders;  he  will  aim  to  prevent  their 
recurrence.  The  habit  of  depending  upon  laxatives  in- 
stead of  general  hygiene  is  to  be  deplored.  Irrigation  of 
the  colon  to  relieve  from  auto-intoxication  is  often  resorted 
to  as  a  part  of  medical  treatment  with  good  effects,  but 
is  not  to  be  advised  so  long  as  attention  to  diet  and  ex- 
ercise can  be  made  to  serve  the  need. 

Obesity. — The  accumulation  of  adipose  tissue  in  burden- 
some and  disfiguring  deposits  is  regarded  as  mirth-provok- 
ing, but  should  be  viewed  with  due  appreciation  of  its 
seriousness.  In  the  light  of  what  has  gone  before,  there 
is  no  escape  from  the  conclusion  that  during  the  period 
of  increasing  weight  the  diet  must  have  been  in  excess  of 
the  requirement.  Yet  when  a  person  has  once  become 
notably  stout  he  is  often  observed  to  be  a  light  eater.  It 
is  likely  to  be  found  that  he  cannot  reduce  his  allowance  of 
food  without  feeling  quite  uncomfortable.  Various  means 
may  be  resorted  to  in  the  attempt  to  abate  the  unwelcome 
condition,  but  frequently  without  success.  Thus  out- 
door exercise,  which,  of  course,  increases  the  metabolism 
and  should  destroy  the  body-fat,  may  stimulate  the  ap- 
petite to  an  extent  which  fully  corresponds  with  the  oxida- 
tion, and  so  defeats  its  own  purpose. 

The  gathering  of  adipose  tissue  under  the  skin  has  an 
effect  somewhat  like  that  of  extra  clothing.  It  is  an 
obstacle  to  heat  loss  and  makes  it  possible  for  the  possessor 
to  maintain  himself  with  less  expenditure  of  fuel.  Hence, 
when  the  protecting  layer  is  once  established  it  becomes 
more  and  more  easy  to  add  to  it.  Its  absence  from  the 
lean  subject  makes  him  a  more  prodigal  dispenser  of  heat 


216  NUTRITIONAL   PHYSIOLOGY 

to  his  environment  and  he  must  presumably  consume  more 
food  to  secure  an  equilibrium.  It  is,  therefore,  much 
harder  to  begin  the  deposit  than  to  nurture  it  when  it 
exists.  A  fat  man  is  like  a  house  with  double  windows — 
the  arrangement  will  save  coal. 

Reduction  of  adipose  tissue  by  fasting  is  possible,  but 
not  popular.  Several  other  methods  have  been  tried. 
The  Banting  system,  formerly  much  in  vogue,  consisted 
essentially  in  a  diet  containing  a  maximum  of  protein. 
It  was  held  that  such  a  diet  would  keep  the  muscles  and 
glands  from  losing  substance,  while  not  promoting  the 
formation  of  fat.  We  have  seen,  however,  that  fat  forma- 
tion from  protein  is  at  least  theoretically  possible.  The 
success  of  the  Banting  treatment  probably  depended  upon 
two  factors:  first,  foods  rich  in  protein  are  satiating,  and 
an  unconscious  cutting  down  of  income  is  therefore  likely; 
second,  the  specific  dynamic  effect  of  so  much  protein 
would  be  expected  to  increase  the  metabolism.  Restric- 
tion of  water  drinking  is  often  recommended.  Laxatives 
are  sometimes  used,  presumably  to  hinder  absorption. 
A  procedure  which  seems  rational  is  the  selection  of  a 
rather  bulky  but  not  highly  nutritious  diet,  including  fruit 
and  the  coarser  vegetables.  By  this  means  hunger  can  be 
fairly  appeased,  while  the  actual  quantity  of  food  entering 
the  circulation  is  not  sufficient  to  maintain  carbon  equi- 
librium. 


CHAPTER  XXIII 
THE  HYGIENE  OF  NUTRITION  (Continued) 

WATER;   MEAT;   SUGAR 

Water. — There  are  particular  foods  which  call  for  indi- 
vidual notice;  one  of  these  is  water.  The  fact  is  already 
familiar  that  this  compound  so  abundant  in  nature  is  also 
the  largest  item  in  the  income  of  the  human  body.  It  is 
an  essential  part  of  all  the  tissues,  and  its  percentage  in 
their  make-up  cannot  be  materially  reduced  while  life 
continues.  It  has  been  described  before  as  an  important 
vehicle  of  excretion,  and  we  have  seen  that  when  it  evapo- 
rates it  often  provides  for  the  removal  of  heat,  which  would 
otherwise  accumulate  in  the  body  to  its  hurt.  When  the 
discharge  of  water  is  unusually  great  the  feeling  of  thirst 
is  roused  and  dictates  a  renewal  of  the  stored  supply. 

We  can  vary  considerably  our  practice  of  water  drinking. 
Hence  this  is  a  matter  often  dealt  with  by  writers  on 
hygiene.  The  common  teaching  is  to  the  effect  that  one 
can  hardly  drink  too  much  water  unless  it  be  at  mealtimes. 
The  beneficial  results  supposed  to  accrue  from  free  drink- 
ing are  assumed  to  include  the  avoidance  of  constipation 
and  the  promotion  of  the  elimination  of  dissolved  waste  by 
the  kidneys  and  possibly  by  the  liver.  There  seem  to  be 
no  reasons  for  doubting  the  soundness  of  these  popular 
ideas.  The  drinking  of  a  great  deal  of  water  is  an  excel- 
lent habit,  and  consumption  of  tea,  coffee,  and  other 
beverages  in  which  water  is  the  principal  constituent  has 
this  in  its  favor — it  leads  people  to  take  much  more  water 
than  they  would  otherwise. 

It  is  probably  correct  to  say  that  the  duties  of  the  kid- 
neys are  made  lighter  when  we  give  them  more  water  to 

217 


218  NUTRITIONAL   PHYSIOLOGY 

excrete.  This  may  be  contrary  to  the  general  impression, 
for  the  temptation  is  to  judge  the  work  of  a  gland  by  the 
volume  of  its  output.  But  in  the  light  of  facts  which 
cannot  be  discussed  here  we  are  led  to  believe  that  con- 
centration rather  than  sheer  amount  of  secretion  is  what 
puts  the  tubules  of  the  kidneys  to  the  severest  test.  They 
act  at  the  greatest  disadvantage  when  required  to  excrete 
a  maximum  of  solids  in  a  minimum  of  water.  The  urine 
almost  always  has  a  concentration  much  higher  than  that 
of  the  blood  from  v/hich  it  is  derived,  and  it  is  fair  to  assume 
that  the  separation  of  the  two  fluids  would  be  made  easier 
if  the  difference  of  concentration  could  be  lessened. 
Water  drinking  is  the  natural  way  to  secure  this  result. 
''Water,"  says  Osier,  ''is,  after  all,  the  great  diuretic." 

While  teachers  of  hygiene  are  well  agreed  upon  the  value 
of  water  in  liberal  quantities  as  conducing  to  health, 
there  has  been  much  said  against  its  free  use  with  meals. 
We  can  hardly  question  the  impression  that  many  cases  of 
indigestion  have  been  benefited  by  the  prohibition  of  water 
at  the  table.  An  approach  to  Fletcherism  is  favored 
when  the  saliva  is  not  aided  by  swallows  of  water.  Slower 
eating  is  likely,  and  slower  eating  may  be  expected  to 
satisfy  the  appetite  with  a  smaller  actual  intake.  The 
idea  that  the  digestive  juices  are  seriously  diluted  by  water 
taken  with  the  food  does  not  seem  to  be  well  founded. 
Laboratory  experiments  show  that  dilution  of  the  fresh 
secretions  is  at  least  as  likely  to  increase  as  to  diminish 
their  activity.  It  must  be  borne  in  mind  that  the  dilution 
of  a  liquid  containing  a  fixed  amount  of  enzyme  does  not 
reduce  the  quantity  of  the  enzyme,  but  only  makes  it  act 
in  a  larger  volume  of  the  mixture. 

Very  cold  water  swallowed  rapidly  may  chill  the  mucous 
membrane  of  the  stomach  and  possibly  retard  the  prog- 
gress  of  gastric  secretion.  Digestion  itself  may  also  be 
slowed  if  the  contents  of  the  stomach  are  cooled.  But 
bacterial  fermentation  should  apparently  be  delayed  just 
as  definitely  as  the  normal  hydrolysis,  and  we  ought  not 
to  make  too  much  of  these  possibilities.     Very  recently 


THE    HYGIENE    OF    NUTRITION  219 

Hawk,  of  the  University  of  Illinois,  has  made  a  number  of 
trials  which  are  entirely  favorable  to  the  practice  of  drink- 
ing all  the  water  that  one  chooses  with  meals.  He  has 
shown  that  the  fecal  nitrogen  is  lower  when  water  is  taken  in 
large  volmne  than  when  it  is  forbidden.  This  fact  he  holds 
to  indicate  more  complete  digestion  and  more  thorough  ab- 
sorption. His  subjects  were  in  the  best  of  health,  and  his 
results  do  not  contradict  those  of  physicians  who  have  found 
the  restriction  of  water  beneficial  in  particular  cases. 

The  opinion  is  commonly  held  that  drinking  much  water 
favors  increase  of  weight.  To  a  limited  extent  such  in- 
crease of  weight  may  result  from  actual  retention  of  water. 
We  can  see  that  a  definite  addition  to  the  adipose 
tissue  may  proceed  from  the  tendency  to  eat  more  food 
when  water  is  taken  freely.  Perhaps  the  converse  of  this 
form  of  statement  is  more  accurate;  namely,  that  weight 
is  lost  when  water  is  restricted  and  the  quantity  of  solid 
food  unconsciously  diminished.  Sometimes  an  erroneous 
inference  may  have  been  drawn  from  the  fact  that  stout 
people  drink  a  great  deal  of  water.  This  is  in  part  a 
consequence  rather  than  a  cause  of  their  condition.  Sub- 
cutaneous fat  is  a  hindrance  to  the  escape  of  heat  from  the 
body,  and  its  presence  during  warm  weather  necessitates 
an  unusual  amount  of  perspiration.  This,  in  turn,  pro- 
duces thirst. 

Meat. — ]\Iuch  that  is  written  tends  to  create  the  im- 
pression that  meat  is  entirely  unlike  any  other  food.  Its 
peculiarities  are  constantly  exaggerated.  When  Erasmus 
Darwin  excused  himself  from  Lenten  abstinence  on  the 
ground  that  ''all  flesh  is  grass''  he  perverted  Scripture,  but 
uttered  a  physiologic  truth.  Meat  has  two  distinctive 
characters:  it  is  very  rich  in  proteins  and  in  extractives. 
It  is  not  any  richer  in  protein  than  are  peas  and  beans,  but 
while  these  vegetables  contain  a  large  proportion  of  carbo- 
hydrates, lean  meat  contains  these  bodies  very  scantily. 
A  diet  of  lean  meat,  therefore,  comes  near  to  being  a 
straight  protein  diet.  A  plate  of  beans  is  equivalent  in 
composition  to  a  plate  of  meat  and  potato. 


220  NUTRITIONAL   PHYSIOLOGY 

Vegetarianism  has  sometimes  been  advocated  upon 
humanitarian  grounds  and  sometimes  because  of  its  sup- 
posed favorable  effect  upon  health.  The  idea  that  the 
destruction  of  animal  life  for  hmnan  nutrition  is  morally 
wrong  cannot  be  discussed  here.  It  is  true  that  not  many 
people  would  kill  animals  for  their  own  use  if  there  were  no 
other  way  to  obtain  meat.  It  is  equally  true  that  so  many 
of  the  noblest  and  gentlest  people  that  ever  lived  have 
enjoyed  meat  and  fish  that  we  cannot  well  credit  the  claim 
that  flesh  foods  cause  deterioration  of  character. 

A  diet  containing  much  meat  is  naturally  a  high  protein 
diet,  and  is,  accordingly,  subject  to  the  drawbacks  already 
mentioned  in  this  connection.  These  have  been  seen  to 
include  an  unprofitable  spurring  of  the  metabolism — ^more 
particularly  objectionable  in  warm  weather — and  the 
menace  of  auto-intoxication.  The  typical  proteins  of  meat 
are  probably  not  better  nor  worse  than  other  proteins  in 
these  relations.  There  is,  however,  greater,  likelihood  of 
overconsumption  of  proteins  when  meat  is  the  source  be- 
cause of  its  very  attractiveness.  Men,  especially  those  who 
spend  money  freely,  are  certainly  prone  to  such  indiscre- 
tions, while  women  set  an  example  of  temperance  in  this  as 
in  so  many  other  practices.  So  far  as  the  proteins  eaten 
are  destined  to  be  reconstructed  into  those  of  the  blood 
and  tissues,  it  may  be  asserted  that  meat  proteins  are  pecu- 
liarly well  suited  to  the  purpose.  Their  molecular  corre- 
spondence with  the  pattern  to  be  imitated  is  close.  But 
the  demand  for  amino-acids  for  this  use  seems  to  be 
small. 

The  real  individuality  of  meat  is  owing  to  the  presence 
in  it  of  the  secondary  substance  which  we  call  extractives. 
These  give  it  its  odor  and,  in  conjunction  with  mineral 
salts,  its  flavor.  Their  absolute  amount  is  not  large,  but 
they  are  of  much  interest.  They  have  been  described  by 
writers  holding  meat  in  disfavor  as  waste-products  of 
animal  life.  The  characterization  seems  broadly  to  be  a 
justifiable  one,  though  our  disgust  at  the  notion  is  not 
necessarily  well  founded.     The  extractives  of  meat  cer- 


THE    HYGIENE    OF   NUTRITION  221 

tainly  originated  during  the  lifetime  of  the  animal  through 
the  breaking  down  of  its  proteins,  and  were  destined  for 
ultimate  excretion,  either  unchanged  or  after  some  altera- 
tion. It  is  hard  to  escape  the  conclusion  that  our  pleasure 
in  the  taste  of  meat  is  due  to  compounds  nearly  akin  to 
those  of  the  urine. 

Furthermore,  it  is  plain  that  the  opponents  of  meat  are 
correct  in  making  the  point  that  when  we  receive  these  ex- 
tractives we  are  simply  adding  to  the  duties  of  our  own  kid- 
neys. When  we  eat  50  grams  of  protein  in  beans  we  have 
later  to  excrete  about  8  grams  of  nitrogen  in  urea  and  other 
forms.  When  we  eat  50  grams  of  protein  in  beefsteak  we 
must  subsequently  excrete  the  same  quantity  of  nitrogen 
plus  that  of  the  extractive  bodies.  The  addition  is  not  a 
large  one,  but  it  is  partly  in  the  form  of  uric  acid  and  per- 
haps other  bodies  less  tractable  than  urea.  The  advisa- 
bility of  limiting  meat  in  rheumatism  and  related  conditions' 
has  long  been  recognized. 

While  the  extractives  may  be  held  to  account  for  some  of 
the  possible  ill  effects  of  meat,  they  are  also  the  source  of  its 
especial  virtue.  Palat ability  itself  is  of  great  hygienic 
worth,  and  these  substances  confer  qualities  which  for 
most  people  cannot  be  equaled  apart  from  meat.  The 
promotion  of  gastric  secretion  in  the  normal  subject  and 
its  establishment  in  the  invalid  are  most  surely  secured  by 
means  of  these  same  extractives.  Aside  from  their  favor- 
able influence  upon  the  stomach,  they  are  probably  mild 
stimulants  in  the  same  sense  in  which  coffee  and  tea  can 
be  called  so. 

Some  kinds  of  meat  are  well  known  to  occasion  indiges- 
tion. Pork  and  veal  are  particularly  feared.  While  we 
may  not  know  the  reason  why  these  foods  so  often  disagree 
with  people,  it  seems  probable  that  texture  is  an  important 
consideration.  In  both  these  meats  the  fiber  is  fine,  and 
fat  is  intimately  mingled  with  the  lean.  A  close  blending 
of  fat  with  nitrogenous  matter  appears  to  give  a  fabric 
which  it  is  hard  to  digest.  The  same  principle  is  illus- 
trated by  fat-soaked  fried  foods.     Under  the  cover  of  the 


222  NUTRITIONAL   PHYSIOLOGY 

fat  thorough-going  bacterial  decomposition  of  the  proteins 
may  be  accomplished  with  the  final  release  of  highly  poi- 
sonous products.  Attacks  of  acute  indigestion  resulting 
from  this  cause  are  much  like  the  so-called  ptomain- 
poisoning.  But  it  is  best  to  reserve  the  term  for  those 
cases  in  which  the  harmful  bacterial  change  had  taken 
place  in  the  food  before  it  was  eaten. 

Texture  is  an  exceedingly  weighty  factor  in  determining 
the  ease  or  difficulty  of  digestion  for  any  food.  It  is  this 
which  makes  the  recognized  difference  between  new  and 
old  bread  and  between  the  upper  and  the  under  crust  of  a 
pie.  Osier  has  said,  with  his  usual  picturesqueness,  that 
"pie  north  of  Mason  and  Dixon's  Line  and  hot  bread  south 
of  it  have  done  more  harm  than  alcohol."  Fats  are  best 
cared  for  when  emulsified  if  liquid,  and  when  of  a  flaky 
or  crystalline  character  if  solid.  This  last  quality  is  real- 
ized in  good  bacon. 

Sugar. — The  reader  will  have  noted  that  the  starch, 
which  is  usually  the  most  abundant  compound  in  the  daily 
income  of  man,  is  converted  to  a  sugar  before  it  is  admitted 
to  the  circulation.  The  question  arises  why  it  is  not 
equally  well  to  eat  sugar  altogether  in  place  of  starch.  This 
is  the  actual  habit  during  the  period  of  milk  feeding  in  in- 
fancy. The  enzymes  that  act  upon  starch  are  not  freely 
furnished  until  some  months  after  birth.  In  later  life, 
however,  starch  comes  to  be  the  main  reliance  of  the  race. 
Experience  has  shown  that  cane-sugar  is  often  productive 
of  indigestion.  Can  we  find  definite  reasons  for  the 
superiority  of  starch  to  sugar? 

The  fact  has  previously  been  mentioned  that  much  sugar 
causes  alimentary  glycosuria,  while  this  is  never  produced 
by  the  freest  eating  of  starch.  Here  we  have  a  clue  to  the 
difference  between  the  two  classes  of  carbohydrates. 
The  glycosuria  is  not  in  itself  a  serious  matter,  but  it 
shows  that  the  solubility  of  sugar  and  the  slight  changes 
requisite  for  its  digestion  lead  to  its  very  rapid  absorption. 
The  digestion  of  starch  is  a  more  gradual  and  protracted 
process,  and  the  resulting  glucose  is  not  formed  so  swiftly 


THE    HYGIENE    OF    NUTRITION  223 

as  to  raise  the  concentration  of  the  intestinal  contents 
appreciably,  nor  delivered  to  the  blood  so  abruptly  as  to 
increase  markedly  the  percentage  of  sugar  in  circulation. 
It  seems  safe  to  assume  that  the  storage  of  glycogen  is 
effected  more  smoothly  and  easily  after  the  ingestion  of 
starch  than  after  the  taking  at  one  time  of  a  large  quantity 
of  sugar. 

Highly  soluble  bodies  of  low  or  moderate  molecular  weight 
are  said  to  confer  on  their  concentrated  solutions  the  prop- 
erty of  high  osmotic  pressure.  This  is  not  the  place  to 
discuss  what  is  meant  by  the  expression.  For  practical 
purposes  it  can  be  said  that  concentrated  solutions  take 
water  from  tissues  with  which  they  may  be  brought  in 
contact.  Hence  an  irritant  effect  is  to  be  expected.  This 
is  exemplified  by  the  action  of  strong  salt  solutions  in 
producing  vomiting.  Everyone  who  is  fond  of  candy 
knows  that  it  can  be  eaten  until  a  point  is  reached  at 
which  an  uneasy  sensation  of  satiety  verging  on  nausea 
is  developed.  This  is  relieved  by  drinking  water,  which,  of 
course,  lowers  the  concentration  of  the  syrupy  gastric 
contents  and  so  lessens  the  irritation. 

Cane-sugar  is  much  sweeter  than  sugar  of  milk  or  the 
other  sugars  which  arise  in  the  course  of  digestion.  It  is, 
therefore,  cloying  and  its  free  use  may  blunt  the  appetite 
for  other  foods.  It  is  said  also  to  be  more  disturbing  to  the 
stomach  than  the  others.  These  two  properties  perhaps 
account  sufficiently  for  the  ill  effects  which  are  attributed 
to  the  increasing  consumption  of  candy  in  this  country. 
Still  it  is  probable  that  the  evils  resulting  have  been  much 
exaggerated.  There  is,  however,  no  question  that  the 
constant  eating  of  candy  threatens  to  damage  the  teeth, 
and  the  indirect  impairment  of  the  digestive  powers  may  be 
serious. 

The  relation  between  sugar  and  the  decay  of  the  teeth 
is  apparently  quite  simple.  Sugar  is  prone  to  ferment, 
a  change  brought  about  by  bacteria  which  are  inevitably 
present  in  the  mouth.  The  chief  product  of  such  bacterial 
decomposition  is  lactic  acid.     This  acid  attacks  the  lime- 


224  NUTRITIONAL   PHYSIOLOGY 

salts  of  the  teeth  at  points  where  the  enamel  has  been 
chipped  or  worn  away,  or  where  it  fails  to  meet  the  gum. 
The  dissolving  of  the  lime-salts  leaves  a  soft  and  perishable 
organic  structure  which  readily  undergoes  true  decay. 
Every  tiny  deposit  of  sugar  in  the  crevices  of  the  teeth  may 
soon  become  a  focus  of  acid  production  and  a  center  of 
disintegration.  The  popular  impression  that  plain  sugar 
is  not  so  hurtful  as  sugar  mingled  with  other  substances  in 
candy  has  this  basis :  pure  sugar  is  so  readily  dissolved  by 
the  saliva  that  it  is  not  likely  to  remain  long  clinging  to  the 
teeth.  On  the  other  hand,  sugar  which  is  mixed  with  fatty 
materials  like  chocolate  may  be  sealed  into  crannies  or 
retained  under  the  edge  of  the  gum  with  unfortunate  effect. 
One  who  is  bound  to  eat  much  candy  should  be  willing  to 
exercise  unusual  care  to  free  the  teeth  from  the  remains, 
and  may,  in  spite  of  his  pains,  have  periodical  days  of 
reckoning  at  the  dentist's. 

Food  Accessories. — It  is  perhaps  imnecessary  to  enlarge 
upon  the  service  of  those  compounds  which  are  classed 
under  this  head.  The  double  value  of  the  extractives  of 
meat  which  favor  digestion  both  because  of  the  flavors 
which  they  develop  and  because  of  their  direct  action  upon 
the  stomach  wall  has  been  sufficiently  emphasized.  The 
various  condiments  are  believed  to  have  a  similar  signifi- 
cance. So  far  as  they  season  the  food  so  as  to  make  it 
more  acceptable,  they  must  evidently  promote  secretion. 
Their  power  to  call  forth  the  gastric  juice  by  a  purely  local 
effect  is  not  so  well  established,  but  they  are  known  to  in- 
crease the  blood-flow  in  the  mucous  membrane,  which 
must  help  to  sustain  the  local  activities. 

Tea  and  Coffee. — These  beverages  owe  what  limited 
food  value  they  have  to  the  cream  and  sugar  usually 
mixed  with  them.  They  give  pleasure  by  their  aroma,  but 
they  are  given  a  peculiar  position  among  articles  of  diet  by 
the  presence  in  them  of  the  compound  caffein,  which  is 
distinctly  a  drug.  It  is  a  stimulant  to  the  heart,  the  kid- 
neys, and  the  central  nervous  system.  It  is  chemically 
related  to  uric  acid,  but  is  not  known  to  yield  this  incon- 


THE   HYGIENE  OF   NUTRITION  225 

venient  waste-product  in  the  human  body.  Individual 
susceptibihty  to  the  action  of  caffein  varies  greatly. 
Where  one  person  notices  little  or  no  reaction  after  a  cup  of 
coffee,  another  is  exhilarated  to  a  marked  degree  and  hours 
later  may  find  himself  lying  sleepless  with  tense  or  trem- 
bling muscles,  a  dry,  burning  skin,  and  a  mind  feverishly 
active.  Often  it  is  found  that  a  more  protracted  disturb- 
ance follows  the  taking  of  coffee  with  cream  than  is  caused 
by  black  coffee. 

It  is  too  much  to  claim  that  the  use  of  tea  and  coffee  is 
altogether  to  be  condemned.  Many  people,  nevertheless, 
are  better  without  them.  For  all  who  find  themselves 
strongly  stimulated  it  is  the  part  of  wisdom  to  limit  the 
employment  of  these  decoctions  to  real  emergencies  when 
uncommon  demands  are  made  upon  the  endurance  and 
when  for  a  time  hygienic  considerations  have  to  be  ig- 
nored. If  young  people  will  postpone  the  formation  of  the 
habit  they  will  have  one  more  resource  when  the  pressure 
of  mature  life  becomes  severe. 

Chocolate  and  its  derivative,  cocoa,  may  be  regarded  as 
having  somewhat  similar  properties.  There  is  a  measure 
of  drug  action,  though  it  is  less  pronounced  than  in  the 
case  of  coffee.  Here  the  active  principle  is  theobromin, 
nearly  related  to  caffein.  Chocolate  is  more  than  a  food 
accessory,  being  exceedingly  nutritious.  A  chocolate 
habit  is  easily  formed,  but,  aside  from  threatening  damage 
to  the  teeth,  it  is  comparatively  innocent. 

Mineral  Salts. — These  compounds  have  been  referred 
to  in  an  earlier  chapter  as  forming  an  essential  part  of  the 
tissues.  Hence  they  must  be  supplied  in  sufficient  quan- 
tity and  variety  during  the  period  of  growth.  There  is  no 
real  danger  of  failing  to  do  this,  though  there  are  certain 
cases  of  malnutrition  in  which  the  central  difficulty  seems 
to  be  the  lack  of  such  constituents  in  the  body  of  the 
child.  The  actual  trouble  is  probably  with  the  absorp- 
tion rather  than  with  the  diet.  Thus,  in  rickets  there 
is  an  evident  deficiency  in  the  quantity  of  lime-salts  in- 
corporated into  the  developing  skeleton,  but  it  is  not 

15 


226  NUTRITIONAL   PHYSIOLOGY 

often  true  that  there  is  any  shortage  in  the  amount  offered 
in  the  food. 

When  the  full  stature  is  reached  the  need  of  a  continued 
salt  income  is  less  marked,  though  there  is  reason  to  believe 
that  the  demand  always  exists.  A  certain  loss  in  the  urine 
and  through  the  skin  seems  bound  to  occur,  though  the 
usual  excretion  of  salts  is  far  greater  than  the  bare  mini- 
mum, and  appears  to  indicate  a  needless  excess  in  the  in- 
come. The  salts  of  the  diet  have  much  to  do  with  its 
palatability  and  so  deserve  a  place  with  the  organic  condi- 
ments. An  unusual  quantity  of  saline  matter  may  be  sup- 
posed to  impose  a  hard  task  upon  the  kidneys,  and  is 
known  to  aggravate  any  dropsical  tendency  that  may  be 
present. 

Sodium  chlorid  is  the  one  salt  which  we  take  pains  to 
secure.  We  are  inclined  to  think  of  it  as  a  mere  relish, 
but  it  has  been  shown  that  it  has  a  deeper  significance. 
The  Austrian  physiologist  Bunge  has  found  that  it  is 
sought  both  by  anim_als  and  men  whose  food  is  largely  or 
entirely  vegetable.  It  is  repugnant  to  those  that  are  ap- 
proximately carnivorous.  Bunge  points  out  that  vegetable 
foods  are  generally  rich  in  compounds  of  potassium  and 
relatively  deficient  in  those  of  sodium.  He  has  demon- 
strated that  when  an  excess  of  potassium  salts  is  eaten 
the  kidneys  discharge  the  foreign  material  promptly,  and 
in  doing  so  let  slip  a  good  deal  of  the  sodium  chlorid  from 
the  blood.  Accordingly,  it  is  inevitable  that  the  steady 
consimiption  of  foods  rich  in  potassium  should  create  a 
demand  for  sodium.  The  seeking  of  common  salt  to  meet 
the  need  is  a  singular  illustration  of  the  almost  unerring 
working  of  instinct. 


CHAPTER  XXIV 
ALCOHOL 

Alcohol  occupies  a  peculiar  position  among  the  con- 
stituents of  the  diet  of  mankind.  A  perfectly  dispassionate 
estimate  of  its  values  and  its  drawbacks  is  arrived  at  with 
difficulty.  JNIost  discussions  of  the  subject  are  frankly 
partisan  and,  therefore,  partial.  Alcohol  affects  the 
human  system  in  many  ways,  and  it  is  possible  to  select  for 
emphasis  either  those  aspects  of  its  action  which  are  detri- 
mental or  those  which  are  favorable.  In  this  way  a  writer 
may  be  entirely  accurate  in  all  his  affirmations  and  yet  fail 
to  be  just  to  a  complex  question  because  of  what  he  leaves 
unsaid.  One  cannot  properly  approach  such  an  analysis 
of  the  effects  of  alcohol  without  first  learning  the  extent 
of  its  use  the  world  over.  In  our  own  environment  there 
is  a  feeling  of  hostility  toward  the  intoxicant  which  would 
excite  wonder  in  many  countries  enjoying  an  advanced 
civilization.  Intemperance  is  deplored  by  thoughtful 
people  everyrs'here,  but  the  demand  for  total  abstinence 
is  sectional,  though  probably  extending  steadily. 

The  older  literature  abounds  in  the  praise  of  wine.  The 
Bible  itself  has  many  appreciative  references  to  its  potency 
as  a  comforter.  It  has  also  eloquent  passages  which  con- 
demn its  abuse  and  records  of  abstinence  on  the  part  of 
certain  sects  or  guilds  among  the  Hebrews.  No  better 
physiologic  distinction  has  ever  been  dra"s\Ti  than  in  the 
verse  which  places  'Vine  which  maketh  glad  the  heart  of 
man"  in  comparison  with  "bread  which  strengtheneth 
man's  heart."  To  "make  glad"  is  to  minister  to  feeling, 
to  "strengthen"  is  to  confer  power  which  can  be  demon- 
strated to  an  observer — an  objective  instead  of  a  subjective 

227 


228  NUTRITIONAL    PHYSIOLOGY 

result.  We  cannot  point  to  many  great  men  in  the  his- 
tory of  nations  who  have  entirely  avoided  the  use  of  al- 
cohol. 

Here  in  America  there  was  little  concerted  protest 
against  the  use  of  alcoholic  drinks  until  the  nineteenth 
century.  The  Puritans,  with  all  their  restriction  of  recre- 
ation and  self-indulgence,  were  singularly  tolerant  of  hard 
cider  and  Jamaica  rum.  This  laxity  extended  to  all  classes 
of  society.  About  one  hundred  years  ago  Lyman  Beecher 
described  the  immoderate  drinking  which  was  a  feature  of 
an  ordination  to  the  ministry  in  a  Connecticut  parsonage, 
host  and  guests  being  clergymen.  Beecher  himself  be- 
came a  vigorous  leader  of  the  movement  for  temperance, 
which  was  not  until  some  years  later  an  agitation  for  total 
abstinence. 

Edward  Everett  Hale  has  told  us  in  his  ''A  New  Eng- 
land Boyhood"  of  the  common  practice  of  serving  wine 
at  children's  parties  about  the  year  1830.  He  also 
tells  us  that  when  the  "Franklin  Medals"  were  annually 
awarded  to  Boston  schoolboys,  an  entertainment  and  ban- 
quet was  provided  from  which  the  youths  departed  in  a 
tipsy  condition.  The  utter  impropriety  of  these  proceed- 
ings shows  us  in  an  impressive  manner  how  far  we  have 
moved  from  the  standpoint  of  that  age.  The  offenses  of 
the  time  appear  the  more  aggravated  when  we  reflect  that 
the  liquors  used  were  largely  of  the  strongest  type. 

The  world  has  long  had  one  conspicuous  example  of  con- 
sistent abstinence  on  the  part  of  a  great  population.  This 
is  afforded  by  the  Mahometan  peoples.  It  is  said  that  the 
Lascar  sailors  who  visit  our  ports  can  be  allowed  shore 
leave  with  implicit  confidence  that  they  will  return  to  the 
ship  as  sober  as  when  they  left  it.  A  seaman  who  erred 
in  this  respect  and  was  remonstrated  with  by  his  captain  is 
reported  to  have  excused  himself  on  the  ground  that  he  had 
embraced  Christianity. 

Something  will  be  said  of  alcohol  under  each  of  five 
heads.  We  shall  proceed  to  consider  it  as  a  relish,  a  food, 
a  drug,  a  cerebral  alterative,  and  as  a  poison.     While 


ALCOHOL  229 

such  a  treatment  is  most  convenient,  it  must  be  recognized 
that  alcohol  can  scarcely  exert  a  single  influence  unmixed 
with  the  others.  One  of  the  five  aspects  named  may  be 
particularly  prominent  for  the  moment,  but  traces  at  least 
of  the  others  are  to  be  looked  for.  It  is  this  intricacy  of 
action  which  makes  it  so  hard  to  weigh  the  facts  with 
equity.  Another  disturbing  consideration  is  found  in  the 
very  unequal  susceptibility  of  different  persons  to  the 
temporary  effect  of  alcohol  as  well  as  to  its  habit-forming 
property. 

Alcohol  as  a  Relish. — It  must  be  sufficiently  clear  from 
what  has  gone  before  that  anything  that  adds  to  the  zest 
and  pleasure  of  a  meal  may  be  expected  to  promote  diges- 
tion. The  only  exception  to  this  rule  may  be  looked 
for  when  the  relish  in  some  way  interferes  with  the  di- 
gestive process.  Alcohol  or,  more  correctly,  alcohohc 
beverages  may  certainly  be  held  to  enhance  the  enjoy- 
ment of  dining,  and  must,  therefore,  favor  the  digestion 
and  absorption  of  food.  It  is  often  claimed  in  rebuttal 
that  alcohol  retards  the  action  of  enzymes.  This  is  doubt- 
less true  of  high  concentrations,  but  no  such  mixtures  can 
possibly  be  made  to  exist  in  the  stomach.  Rapid  dilu- 
tion by  the  juices  and  rapid  absorption  of  the  alcohol 
through  the  lining  membrane  combine  to  bring  down  its 
percentage  to  a  level  which  cannot  hinder  the  normal 
hydrolysis. 

The  appeal  of  wines  and  cocktails  may  be  said  to  be 
due  quite  as  much  to  the  minor  substances  which  they 
contain  as  to  the  alcohol.  Pure  dilute  alcohol  would 
not  be  attractive  to  many  people.  At  the  same  time  the 
extractives  conferring  flavor  and  fragrance  do  not  seem 
to  make  a  complete  beverage  when  the  alcohol  is  removed. 
With  the  frank  admission  that  the  finer  alcoholic  drinks 
may  be  promoters  of  digestion,  we  may  fairly  couple 
certain  qualifying  statements.  First,  such  stimulation 
is  unnecessary  for  those  whose  health  is  what  it  should  be. 
Second,  the  employment  of  such  means  to  spur  the  ap- 
petite leads  readily  to  overeating.     It  is  also  to  be  ob- 


230  NUTRITIONAL   PHYSIOLOGY 

served  that  the  use  of  alcoholic  relishes  gives  no  sanction 
to  drinking  apart  from  meals. 

Alcohol  in  the  stomach  has  the  effect  of  increasing  the 
local  blood-flow,  and  it  is  well  established  that  absorp- 
tion is  thereby  promoted.  Whether  it  takes  its  place 
with  the  extractives  or  meat  as  a  chemical  agent  to 
excite  gastric  secretion  is  not  so  certain.  It  is  hard  to 
discriminate  between  the  influence  which  it  exercises 
through  the  central  nervous  system  and  that  which  it  may 
exert  upon  the  stomach  wall  in  a  more  direct  manner. 
The  maximum  favorable  effect  upon  the  digestion  is  pro- 
duced by  a  small  quantity  of  alcohol.  Larger  quantities 
are  notoriously  apt  to  nauseate  and  to  precipitate  dis- 
graceful scenes,  all  too  common  in  connection  with  elab- 
orate banquets. 

Alcohol  as  a  Food. — There  has  been  a  great  deal  of 
opposition  to  the  claim  that  alcohol  can  be  reckoned  a 
food.  It  has  seemed  to  many  of  its  antagonists  that  the 
admission  weakens  their  position,  but  this  is  not  neces- 
sarily the  case.  To  say  that  alcohol  may  be  a  food  is  not 
to  deny  that  it  is  a  dangerous'  one.  Professor  Atwater 
was  roundly  censured  by  leaders  of  the  total  abstinence 
movement  for  a  publication  which  is  really  a  powerful 
tract  in  favor  of  their  position.  He  had  said  that  alcohol 
might  serve  as  a  food,  and  all  his  earnest  warnings  against 
it  were  regarded  as  discounted.  His  experimental  work 
on  the  subject  is  entirely  conclusive. 

The  fact  has  been  reiterated  that  the  chief  use  of  food 
is  to  undergo  oxidation  with  release  of  energy.  Alcohol 
in  considerable  amounts  may  be  oxidized  to  carbon  dioxid 
and  water  in  the  human  system.  When  it  is  thus  oxidized 
the  heat  value  of  each  gram  is  about  7  Calories.  There 
seems  to  be  no  provision  for  the  retention  of  alcohol  against 
a  future  time  of  need  nor  for  its  conversion  into  glycogen 
or  fat.  Its  oxidation  is  bound  to  take  place  with  little 
delay,  and  it  is  in  this  respect  not  so  adaptable  to  the 
changing  demands  of  the  organism  as  is  carbohydrate. 
Nevertheless  we  must  grant  that  it  may  take  its  place 


ALCOHOL  231 

in  the  diet  as  a  substitute  for  other  non-nitrogenous 
foods.  Atwater's  experiments,  as  well  as  many  parallel 
studies,  have  made  this  evident. 

A  subject  was  brought  into  equilibrium  on  a  ration 
without  alcohol.  It  was  then  found  possible  to  withdraw 
carbohydrates  and  fats  to  a  certain  extent  and  to  replace 
with  alcohol  in  isodynamic  amounts  without  disturbing 
the  equilibrium.  The  well-known  fattening  effect  of 
moderate  drinking  may  be  explained  as  due  in  part  to  the 
whetting  of  appetite  and  in  part  to  the  sparing  of  car- 
bohydrate and  fat  by  the  alcohol  which  is  utilized  in  their 
stead.  Fifty  years  ago  the  same  fact  was  recognized  by 
George  Henry  Lewes,  in  his  interesting  ' 'Physiology 
of  Common  Life."  He  tells  us  how  a  convention  of  total 
abstainers  once  gathered  in  the  city  of  Frankfort,  in  Ger- 
many, and  how  the  cooks  in  the  hotel  in  which  the  delegates 
lodged  were  put  to  it  as  never  before  to  supply  the  pastry 
and  pudding  ordered  by  these  unfamiliar  patrons.  The 
guests  for  whom  the  table  had  heretofore  been  set  were 
accustomed  to  supply  with  alcohol  a  want  which  the  tee- 
totalers met  with  carbohydrate. 

How  far  the  substitution  of  alcohol  for  other  non- 
nitrogenous  foods  may  be  carried  has  been  much  debated. 
If  it  is  given  too  freely  its  oxidation  is  incomplete  and, 
what  is  of  more  practical  importance,  the  cerebral  effect 
becomes  prominent.  To  make  the  utmost  use  of  alcohol 
as  a  nutrient  it  must  be  taken  in  small  quantities  and 
frequently.  There  seems  to  be  a  general  agreement  that 
from  50  to  75  grams  of  alcohol  in  twenty-four  hours  is 
about  as  much  as  can  be  allowed  to  an  adult  without  un- 
toward reactions.  If  we  assume  the  larger  amount  to  be 
permissible,  it  follows  that  alcohol  may  furnish  as  much  as 
500  calories,  or  about  one-fifth  of  the  day's  total.  The 
requirement  may  be  translated  into  terms  of  various  bev- 
erages: whisky,  something  less  than  J  pint;  sherry  or  port, 
1  pint;  champagne,  1  quart;  beer,  2  quarts. 

Meltzer  has  said  that  "alcohol  in  health  is  mostly  a 
curse  and  in  sickness  mostly  a  blessing."     Its  peculiar 


232  NUTRITIONAL   PHYSIOLOGY 

merits  in  illness  are  connected  with  the  fact  of  its  appetizing 
character  and  with  the  circumstance  that  it  requires  no 
digestion.  In  this  it  resembles  glucose.  When  alcohol  is 
given  to  a  patient  no  call  is  made  upon  the  digestive  glands. 
Hence  it  may  be  tolerated  and  absorbed  when  most  foods 
would  remain  undigested.  So  far  as  it  can  be  introduced 
into  the  circulation  at  such  a  time  it  becomes  a  source  of 
heat  and  spares  the  dwindling  stores  of  the  system,  but 
there  is  an  obvious  tendency  on  the  part  of  physicians  to 
restrict  the  use  of  alcohol  in  sickness.  Statistics  from  rep- 
resentative hospitals  show  a  marked  shrinkage  in  the  con- 
sumption of  liquors  during  the  last  few  years. 

Alcohol  as  a  Drug. — Certain  properties  of  alcohol  may  be 
conveniently  brought  under  this  head,  although  no  sharp 
line  of  demarcation  can  be  drawn  between  these  qualities 
and  others  to  be  dealt  with  later.  The  drug  effect  is 
obtainable  from  rather  large  single  doses  in  contrast  to  the 
nutritive  effect  which  was  said  above  to  be  best  secured 
by  small  amounts  given  at  intervals.  The  most  striking 
reaction  of  the  system  to  alcohol  in  doses  which  may  be 
regarded  as  having  the  drug  effect  is  manifested  by  the  cir- 
culatory apparatus.  The  taking  of  a  glass  or  two  of  wine, 
especially  on  an  empty  stomach,  will  usually  cause  increased 
heart  action  and  a  flushing  of  the  skin,  accompanied  by  the 
subjective  impression  of  warmth.  This  bringing  of  more 
blood  to  the  surface  of  the  body  may  be  expected  to  lessen 
the  volume  of  blood  passing  through  the  internal  organs. 

The  chief  value  of  alcohol  as  a  drug  is  connected  with 
this  tendency  to  abate  internal  congestions.  The  central 
fact  in  the  process  which  we  call  ''taking  cold"  is  an  excess 
of  blood  in  the  mucous  membranes.  This  may  be  con- 
tinued for  a  time  without  ill  consequences,  but  is  always  a 
menacing  condition,  lowering  the  local  resistance  to  in- 
fection and  so  inviting  disease.  Alcohol  has  been  much 
used  to  ''break  up"  incipient  colds  and  with  very  good 
success.  Its  influence  upon  the  distribution  of  the  blood 
is  not  unlike  that  of  quinin  and  some  other  drugs;  it  is  also 
similar  to  the  action  of  hot  applications  to  the  skin. 


ALCOHOL  233 

The  supposed  warming  power  of  alcohol  needs  critical 
examination.  It  is  a  fact  not  generally  recognized  that  we 
have  no  reliable  sensations  indicative  of  the  true  body  tem- 
perature. We  realize  only  the  surface  conditions.  A 
man  who  is  out  in  the  cold  may  produce  a  sense  of  comfort 
by  taking  alcohol,  which  will  send  more  blood  into  the 
cutaneous  vessels,  but  this  pleasant  glow  is  the  sign  that 
heat  is  passing  from  the  body  to  its  surroundings.  It  may 
be  purchased  at  the  cost  of  an  expenditure  which  is  im- 
prudent. Arctic  explorers  seem  well  agreed  that  depend- 
ence on  alcohol  during  exposure  to  intense  cold  is  unwise, 
and  that  it  is  better  to  suffer  a  greater  degree  of  discomfort 
than  to  rely  upon  its  delusive  support.  This  must  be  most 
distinctly  the  case  when  the  hardships  are  to  be  borne  for 
an  indefinite  period.  It  is  more  rational  to  give  alcohol 
after  exposure  to  wet  and  cold  than  during  the  trial. 
Afterward  it  may  help  to  readjust  the  circulation  and  to 
ward  off  possible  evil  results. 

Alcohol  as  a  Cerebral  Alterative. — After  all,  the  main 
reason  why  humanity  clings  to  alcohol  and  is  with  so 
much  difficulty  won  over  to  abstinence,  is  found  in  its 
singular  influence  upon  temperament.  This  is  at  the 
root  of  its  social  employment.  The  rather  awkward 
term,  "cerebral  alterative,"  has  been  chosen  to  avoid  the 
more  familiar  but  questionable  name  of  stimulant.  Much 
discussion  has  been  carried  on  concerning  the  right  of  al- 
cohol to  this  designation.  Stimulant  and  narcotic  are 
opposing  terms.  Alcohol  in  large  quantities  is  clearly  a 
narcotic;  whether  it  is  invariably  so  is  a  subject  of  lively 
debate.  It  might  be  supposed  that  there  would  be  no 
difficulty  in  deciding.  Observation  of  men  slightly  af- 
fected by  wine  shows  them  to  be  animated  and  talkative; 
the  natural  verdict  would  be  that  they  exemplify  stimula- 
tion. Yet  all  the  results  of  taking  alcohol  can  be  explained 
upon  the  theory  that  it  is  a  narcotic.  To  show  how  this 
may  be  it  is  only  necessary  to  point  out  that  many  opera- 
tions of  the  nervous  system  are  normally  inhibitory  in 
nature.     When  a  reticent  man  becomes  garrulous,  it  need 


234  NUTRITIONAL   PHYSIOLOGY 

not  be  inferred  that  he  is  stimulated  in  the  best  sense  of 
the  word;  it  may  be  more  accurate  to  say  that  an  agent 
which  is  essentially  depressing  in  its  influence  has  attacked 
first  of  all  the  inhibitory  centers.  Loss  of  self -conscious- 
ness is  an  obvious  feature  of  the  reaction,  and  loss  of  self- 
respect  is  reached  by  an  easy  transition. 

A  genuine  stimulant  should  be  an  aid  to  application. 
Alcohol,  on  the  contrary,  is  hostile  to  perseverance.  Our 
word  dissipation,  which  we  use  for  intemperance,  is  a  very 
suggestive  one.  Scattering  rather  than  concentration  is 
the  essence  of  the  mental  state  produced.  A  keen  thinker 
has  said  that  we  tacitly  contrast  alcohol  with  coffee,  a 
recognized  stimulant,  when  we  acknowledge  the  difference 
between  the  feelings  with  which  we  should  view  the  use  of 
one  and  the  other  by  the  engineer  of  a  limited  train.  We 
instinctively  feel  that  coffee  will  favor  his  unswerving  at- 
tention to  duty  and  that  alcohol  will  make  him  less  reli- 
able. Alcohol  in  small  quantities  leads  to  inconsequence 
in  thinking  and  is  a  handicap  to  any  steady  pursuit. 
On  the  other  hand,  the  facile  changes  in  the  currents  of 
mental  life,  the  lightness  and  the  unexpectedness  of  one's 
remarks,  may  promote  social  ease  and  the  powder  to  be 
entertaining.  If  alcohol  is  the  foe  of  application  it  is  also 
the  foe  of  prosiness. 

Too  much  emphasis  can  scarcely  be  brought  to  bear 
on  the  wide  disparity  between  what  a  man  thinks  that 
alcohol  does  for  him  and  what  an  impartial  study  shows 
that  it  really  does.  That  'Svine  is  a  mocker"  is  a  shrewd 
observation.  The  subjective  impression  is  often  of  an  ex- 
altation of  capacity  which  objective  testing  fails  utterly 
to  confirm.  A  subject  does  certain  problems  before  taking 
a  drink  of  whisky  and  comparable  ones  afterward.  He 
says  that  the  second  task  was  done  with  greater  speed  and 
with  a  nonchalant  confidence  in  his  results.  The  watch 
says  that  he  was  slower,  and  checking  up  the  work  shows 
that  the  errors  were  more  numerous.  It  is  evident  that  his 
judgment  of  his  own  performances  is  unreliable.  This  is 
true  also  of  manual  operations.     The  recent  tests  made 


ALCOHOL  235 

upon  German  type-setters  have  shown  that  speed  and 
accuracy  are  both  made  to  suffer  when  alcohol  has  been 
taken  even  in  very  limited  amounts.  Here,  again,  the 
subjects  have  an  impression  of  their  own  superior  accom- 
plishment under  alcohol,  which  turns  out  to  be  erroneous. 

It  has  been  freely  admitted  above  that  a  little  alcohol 
may  promote  sociability,  but  there  can  be  no  question 
that  the  reputation  which  alcohol  possesses  of  bringing  out 
the  best  wit  and  humor  of  which  men  are  capable  is  largely 
unfounded.  The  reason  is  not  far  to  seek.  This  reputation 
rests  on  the  reports  of  men  who  were  themselves  influenced 
by  the  same  agent  which  was  working  upon  the  nervous 
systems  of  the  speakers  to  whom  they  were  listening.  Their 
reminiscences  are  to  be  taken  with  more  than  a  grain  of 
salt.  The  auditor  who  is  "vinously  exalted" — to  use  a 
phrase  of  Holmes — is  an  exceedingly  lenient  critic.  He 
applauds  with  delight  sallies  which  a  neighbor,  who  has 
turned  down  his  glass,  perceives  to  be  inane,  if  not  in  bad 
taste. 

The  justification  of  the  social  use  of  alcohol  must  be 
based  on  its  power  to  produce  this  singular  state  of  mind. 
It  removes  the  consciousness  of  fatigue  and  the  feeling 
of  care.  The  attention  is  limited  to  the  present  moment 
and  immediate  interests.  The  faculty  of  discrimination 
is  dulled,  and  with  the  consequent  lowering  of  esthetic 
and  intellectual  ideals  there  comes  a  bland  self-satisfaction 
and  a  naive  admiration  of  one's  fellows.  A  vigorous 
writer  has  called  this  process  "drugging  for  delectation." 
Can  such  an  artifice  be  defended?  It  is  most  diflacult  to 
answer  this  question  with  entire  justice  to  both  sides. 
Perhaps  it  may  be  impossible  to  answer  it  in  sweeping 
fashion  for  all  men.  One  who  is  cynical  and  pessimistic 
by  nature  may  really  view  his  affairs  more  justly  and  judge 
his  neighbors  more  equitably  while  under  the  influence  of 
wine.  This  may  be  true  of  other  temperaments,  the  neu- 
rasthenic, for  instance.  But  the  optimist — and  may  we 
not  say  the  normal  individual? — is  not  likely  to  be  bettered 
by  such  an  agent.     Increased  buoyancy  and  good  humor 


236  NUTRITIONAL   PHYSIOLOGY 

in  such  subjects  means  silliness.  If  it  is  true,  as  is  claimed, 
that  gatherings  of  total  abstainers  are  comparatively  dole- 
ful, the  lesson  may  be,  not  that  alcohol  is  necessary  to  good 
fellowship,  but  rather  that  the  average  nervous  system 
is  below  par.  We  doubt  whether  a  man  ought  to  rest 
content  with  any  lower  measure  of  health  than  that  which 
will  insure  the  social  virtues  without  chemical  aid. 

With  advancing  age  it  may  be  unreasonable  to  demand 
so  high  a  standard.  As  infirmities  increase  there  must 
usually  come  a  time  when  comfort  rather  than  efficiency 
is  to  be  sought.  When  it  is  clear  that  this  time  has 
arrived  there  is  much  to  be  said  in  favor  of  the  more  or 
less  regular,  moderate  use  of  alcohol.  It  is  a  great 
anodyne.  Granting  this,  we  may  also  point  out  that  the 
beneficent  effect  in  age  will  be  more  surely  obtained  by 
those  who  have  not  exhausted  the  consolations  of  alcohol 
in  earlier  years. 

Alcohol  as  a  Poison. — It  seems  hardly  necessary  to  en- 
large upon  the  poisonous  properties  of  alcohol.  That  these 
have  been  ridiculously  exaggerated  is  obvious;  that  they 
are  very  real  is  equally  clear.  The  spectacle  of  drunkenness 
and  the  shame  and  misery  that  attend  it  are  too  familiar. 
No  one  who  begins  to  use  alcohol  can  be  quite  sure  that  he 
will  continue  within  bounds.  The  temperate  lives  of  his 
relatives  cannot  be  held  to  prove  him  secure.  Suscepti- 
bility to  the  tendency  to  increase  the  indulgence  is  found 
again  and  again  in  young  men  of  clean  heredity  and  fine 
gifts.  Hence  the  only  absolute  safety  is  in  total  abstinence. 
Yet  the  chances  do  not  favor  the  ruin  of  the  average  man 
who  adds  alcohol  to  his  diet. 

Aside  from  the  habit-forming  property,  it  is  becoming 
more  and  more  widely  recognized  that  alcohol  often  im- 
pairs the  health  of  men  who  cannot  be  charged  with  in- 
temperance. It  predisposes  to  diseases  of  the  heart, 
liver,  and  kidneys.  It  notoriously  lessens  the  chance  of 
survival  when  the  user  contracts  pneumonia.  It  makes 
him  an  unfavorable  subject  for  surgical  operations.  By 
hastening  the  development  of  arteriosclerosis  it  shortens 


ALCOHOL  237 

the  period  of  active  and  effective  life.  Insurance  exam- 
iners are  glad  when  they  can  record  of  an  applicant  that 
he  is  a  total  abstainer. 

Enough  has  been  said  to  show  how  various  are  the 
aspects  of  alcohol.  It  has  been  easy  to  treat  them  sep- 
arately in  the  preceding  paragraphs,  but  no  such  separa- 
tion is  possible  in  practice.  The  undoubted  value  of  the 
alcoholic  relish,  its  occasional  merit  as  a  significant  part 
of  the  ration,  and  even  its  virtue  as  a  drug  cannot  be  util- 
ized without  some  experience  of  its  cerebral  effect  and  the 
risk,  not  always  remote,  of  forming  a  habit.  The  hygienic 
ideal  to  be  striven  for  is  a  robustness  of  life  which  shall 
make  alcohol  superfluous  as  relish,  food,  or  drug,  and  a 
cheerful,  active  mind  which  needs  no  artificial  aid  to  keep 
it  hopeful  and  sympathetic.  The  attainment  may  not  be 
an  easy  task.  Grief  and  worry  and  overwork  may  be 
added  to  an  original  depression  of  temperament,  but  the 
use  of  alcohol  is  never  more  unsafe  than  when  sorrows  are 
the  excuse,  and  never  so  selfish  and  cowardly  as  when  the 
motive  is  to  shun  responsibilities  that  ought  to  be  faced. 
Men  do  not  often  see  the  sinister  suggestion  in  the  high 
spirits  of  one  who  has  forgotten  his  cares  for  an  evening  by 
the  most  moderate  indulgence.  They  fail  to  see  that  the 
banishment  of  the  sense  of  pressing  duties  is  the  very 
characteristic  of  the  drunkard  when,  developed  to  a  logical 
extreme,  it  makes  him  indifferent  to  every  obligation  of 
conscience  and  of  love. 


CHAPTER  XXV 
INTERNAL  SECRETION 

In  an  early  chapter  of  this  book  it  was  stated  that  there 
are  two  ways  in  which  one  organ  of  the  body  may  exert 
an  influence  upon  another.  The  more  familiar  and  more 
studied  method  has  been  through  the  nervous  connections 
which  are  maintained  between  all  organs  and  the  brain 
and  cord.  The  existence  of  such  ties  provides  for  the 
possibility  of  reflex  action.  By  this  means  any  part  of  the 
organism  may  modify  the  behavior  of  any  other  part,  not 
by  affecting  it  directly,  but  by  stimulating  or  inhibiting 
the  centers  which  preside  over  the  second  organ.  Some 
account  of  the  work  of  the  central  nervous  system  is  to 
be  given  hereafter.  Before  this  is  undertaken  it  is  well  to 
pay  some  attention  to  the  other  way  in  which  co-ordination 
is  promoted;  namely,  by  the  transfer  of  chemical  products 
from  place  to  place  through  the  agency  of  the  circulation. 

This  is  the  subject  usually  covered  by  the  term  internal 
secretion.  The  word  hormone  has  been  used  elsewhere  to 
denote  an  active  substance  generated  in  one  place,  but 
destined  to  take  effect  in  another.  The  secretin  produced 
in  the  walls  of  the  upper  part  of  the  small  intestine  and 
carried  thence  to  the  pancreas  and  the  other  digestive 
glands  to  excite  them  to  pour  out  their  juices  is  an  example. 
So  also  is  the  contribution  made  by  the  pancreas  to  the 
blood,  which  proves  to  be  so  important  to  the  utilization  of 
sugar.  In  this  case  it  seems  to  be  chiefly  in  the  muscles 
that  the  hormone  is  valuable.  Other  instances  of  similar 
interaction  can  now  be  given,  and  there  is  reason  to  an- 
ticipate that  the  list  of  internal  secretions  will  soon  be 
made  longer  than  it  is  at  present. 

238 


INTERNAL   SECRETION  239 

There  are  several  organs  once  regarded  as  insignificant 
which  are  now  recognized  as  vital  to  the  welfare  of  the 
whole  system.  Among  these  the  thyroid  gland  has  at- 
tracted particular  interest.  This  is  a  small  bilobed  mass  of 
tissue  situated  in  front  of  the  trachea  below  the  larynx. 
It  is  one  of  several  organs  sometimes  called  ductless  glands. 
The  term  is  more  appropriate  in  this  case  than  in  some 
others,  since  the  microscope  shows  that  the  arrangement 
of  the  cells  is  distinctly  glandular.  They  surround  small 
recesses  such  as  in  typical  glands  would  be  in  communica- 
tion with  an  outlet  or  duct.  Here,  however,  these  cavities 
are  blind.  They  are  seen  to  contain  a  viscid  material,  the 
so-called  colloid  substance,  which  is  evidently  the  product 
of  the  secreting  cells.  Since  there  is  no  channel  leading  to 
the  exterior,  the  only  possibility  is  that  the  distinctive 
secretion  shall  enter  the  circulation  either  directly  or  by 
way  of  the  lymph. 

The  thyroid  gland  is  frequently  enlarged  and  then  gives 
rise  to  the  disfiguring  swelling  known  as  a  goiter.  Such 
enlargements  are  not  necessarily  attended  with  general 
disturbances  of  health,  yet  in  many  cases  there  are  symp- 
toms which  can  be  referred  to  an  excess  of  the  active 
product.  Palpitation,  breathlessness,  extreme  nervous- 
ness, and  marked  loss  of  weight  are  likely  to  be  observed. 
The  same  manifestations  follow  the  giving  of  overdoses  of 
thyroid  extract.  This  has  been  employed  for  the  correc- 
tion of  obesity,  but  it  seems  unwise  to  resort  to  a  drug 
so  powerful  and  far  reaching  in  its  action  for  the  treat- 
ment of  this  condition. 

Just  as  there  may  be  too  much  of  the  thyroid  material 
for  the  good  of  the  subject,  so  there  may  be  a  serious  de- 
ficiency. The  relative  failure  of  the  gland  to  function  as 
it  should  is  the  cause  of  a  definite  disease  in  human  sub- 
jects, and  its  removal  from  dogs  is  followed  by  a  decline 
if  not  by  death.  Young  and  growing  animals  are  most 
seriously  affected.  All  the  facts  which  have  been  gathered 
support  the  belief  that  development  and  general  well- 
being  depend  to  a  considerable  extent  on  the  normal 


240  NUTRITIONAL  PHYSIOLOGY 

functioning  of  the  thyroid.  Very  recently  it  has  been 
shown  that  what  has  been  called  the  thyroid  is,  in  reality, 
a  compound  structure.  In  addition  to  the  type  of  tissue 
which  forms  the  main  mass  of  the  organ  in  man,  there  are 
four  nodules  of  a  different  sort,  the  parathyroids.  These 
undoubtedly  have  a  chemistry  of  their  own  and  a  distinc- 
tive relation  to  the  economy  of  the  body.  For  a  statement 
of  their  peculiar  importance  reference  must  be  had  to 
larger  works  on  physiology. 

The  full  measure  of  the  influence  which  radiates  from 
the  thyroid  can  be  appreciated  by  considering  the  condi- 
tion known  as  cretinism.  This  is  the  term  used  to  de- 
scribe the  state  of  individuals  in  whom  the  thyroid  has 
never  performed  its  proper  work.  These  subjects  remain 
for  years  in  a  condition  of  arrested  progress,  both  physical 
and  mental.  They  are  uncouth  dwarfs  with  large  heads, 
slack-walled  abdomens,  and  feeble  limbs.  If  they  survive 
to  the  age  of  twenty  or  thirty  they  will  scarcely  have  ad- 
vanced beyond  the  stage  reached  at  four  or  five.  That 
the  lack  of  the  thyroid  is  actually  responsible  for  these 
shocking  cases  is  now  abundantly  proved.  The  demonstra- 
tion is  found  in  the  happy  circumstance  that  great  im- 
provement follows  the  judicious  feeding  of  thyroid  prepa- 
rations to  cretins.  The  material  is  obtained  from  calves 
or  sheep.  Its  administration  for  a  few  months  often  re- 
sults in  transforming  a  repulsive  cretin  into  a  presentable 
child  with  a  prospect  of  at  least  a  moderate  mental  de- 
velopment. 

In  the  dog  it  is  possible  to  graft  an  extra  thyroid  into 
the  abdominal  cavity  and  then  to  remove  the  original 
gland,  when  the  second  organ  will  assume  the  functions 
necessary  to  the  preservation  of  health.  This  was  an 
important  discovery,  since  it  removed  the  ground  for  a 
prevalent  opinion  that  the  service  of  the  thyroid  was 
limited  to  the  reflexes  which  it  was  supposed  to  originate. 
It  had  been  thought  that  the  gland  affected  the  nutrition 
and  general  health  by  sending  impulses  along  the  nerves 
leading  from  it  to  the  centers.     A  gland  substituted  for  the 


INTERNAL   SECRETION  241 

native  one  and  placed  in  a  remote  part  of  the  body  could 
not  be  in  connection  with  the  old  afferent  pathways,  and 
whatever  favorable  effect  it  might  have  must  be  chemical 
in  its  origin. 

There  has  been  the  same  discussion  as  to  whether  the 
reproductive  organs  exercise  their  well-recognized  influence 
on  growth  by  nervous  or  by  chemical  means.  The  strange 
modifications  of  the  type  which  are  produced  as  a  result 
of  castration  are  familiar.  The  differences  in  build  and 
temperament  between  the  unruly  bull  and  the  tranquil  ox 
illustrate  the  consequences.  Such  peculiarities  might  be 
referred  to  reflex  changes  due  to  the  removal  of  sources 
of  stimuli,  but  the  present  tendency  is  to  regard  them  as 
due  chiefly  to  the  loss  of  active  internal  secretions.  The 
case  of  the  pancreas  reminds  us  that  an  organ  may  well  give 
rise  simultaneously  to  products  which  are  permanently  sep- 
arated from  the  blood  and  to  others  which  return  to  it. 
Twenty  years  ago  Brown-S^quard  created  a  furore  among 
people  given  to  premature  acceptance  of  extravagant 
claims  when  he  announced  that  great  rejuvenating  virtues 
could  be  demonstrated  to  exist  in  the  extracts  of  animal 
reproductive  glands.  His  so-called  "Elixir  of  Life"  was  a 
pulp  formed  from  crushed  testes  of  sheep.  At  some  peril 
of  infection  he  injected  this  unsterilized  mixture  under  his 
skin  and  into  the  bodies  of  other  aged  volunteers.  A  cer- 
tain degree  of  stimulation  was  noted,  but  it  has  usually 
been  referred  to  suggestion. 

Since  animals  and  men  are  at  their  best  physically  and 
intellectually  when  the  reproductive  organs  are  active, 
the  expectation  entertained  was  not  wholly  unreasonable. 
Yet  it  was  not  to  be  supposed  that  their  decline  was  re- 
sponsible for  all  the  losses  incident  to  age.  No  extensive 
use  of  such  extracts  has  followed  the  pioneer  work  of 
Brown-Sequard.  The  corresponding  preparations  from 
the  female — ovarian  extracts — have  a  better  standing  in 
modern  medicine.  Given  after  surgical  removal  of  the 
ovaries  they  greatly  relieve  many  of  the  symptoms  which 
commonly  annoy  the  patient. 
16 


242  NUTRITIONAL  PHYSIOLOGY 

The  adrenal  bodies  are  also  numbered  among  the  organs 
which  help  to  maintain  the  system  in  normal  working  order. 
These  are  two  inconspicuous  structures  placed  one  above 
each  kidney.  Their  microscopic  features  are  obscure  and 
do  not  indicate  a  glandular  organization.  From  the  ad- 
renal bodies  can  be  obtained  a  powerful  drug-like  com- 
pound, adrenalin,  which  the  living  cells  of  these  organs  may 
be  supposed  to  deliver  continually  to  the  passing  blood. 
Its  presence  in  the  circulation  is  evidently  a  necessity. 
Destruction  of  the  adrenals — ^which  sometimes  results 
from  local  tuberculosis — produces  the  singular  fatal  dis- 
turbance known  as  ''Addison's  bronze  disease."  An  odd 
feature  of  this  condition  is  the  dark  pigmentation  of  the 
skin,  occurring  sometimes  uniformly  and  sometimes  in 
patches.  More  important,  however,  is  the  steady  loss 
of  vigor  affecting  all  the  contractile  tissues  and  leading  to 
death. 

Adrenalin  injected  into  the  blood-stream  of  an  animal 
shows  its  most  marked  effect  in  the  intense  contraction  of 
the  small  blood-vessels  which  is  produced.  This  property 
has  made  the  substance  valuable  to  surgeons,  since  it  can 
be  used  to  check  bleeding  from  cut  surfaces.  In  like  man- 
ner it  can  reduce  congestion  in  inflamed  tissues,  for  example, 
in  a  blood-shot  eye.  These  artificial  uses  of  adrenalin  can 
all  be  connected  with  its  natural  service,  which  is  largely 
in  the  direction  of  a  reinforcement  of  contractility.  Can- 
non has  recently  called  attention  to  this  fact  in  an  unex- 
pected and  interesting  relation. 

He  has  proved  by  delicate  tests  that  the  blood  of  an 
animal  receives  additional  adrenalin  during  a  terrifying 
experience.  For  example,  he  has  found  the  active  com- 
pound increased  in  the  blood  of  a  cat  which  has  been  kept 
under  restraint  in  the  presence  of  a  barking  dog.  There- 
fore it  must  be  concluded  that  one  of  the  many  bodily 
accompaniments  of  an  emotional  outbreak  is  a  stimulation 
of  the  adrenal  bodies  to  unusual  activity.  The  value  of 
this  reaction  has  just  been  made  clear  through  the  further 
researches   of   the   same   investigator.     It   appears   that 


INTERNAL   SECRETION  243 

extra  resistance  to  fatigue  is  conferred  upon  the  skeletal 
muscles  when  adrenalin  is  sent  to  them.  Thus,  in  an 
emergency  that  might  call  either  for  flight  or  conflict  the 
animal  is  prepared  for  a  maximum  output  of  energy.  We 
have  here  a  scientific  conception  of  the  "strength  of  des- 
peration." 

The  influence  of  other  organs  than  those  mentioned  upon 
the  welfare  of  the  organism  as  a  whole  is  constantly  being 
studied.  A  great  deal  of  attention  is  being  given  to  the 
very  small  but  distinctly  compound  structure,  called  the 
hypophysis,  which  is  lodged  beneath  the  brain  in  a  hollow 
of  one  of  the  cranial  bones.  Its  relation  to  the  course  of  the 
metabolism  is  far  from  simple,  but  it  seems  to  be  clear  that 
it  is  an  indispensable  contributor  to  the  circulating  medium. 
A  much  larger  and  more  conspicuous  affair,  anatomically 
speaking,  is  the  thymus  of  young  animals,  a  mass  of  cells 
lying  back  of  the  breast-bone.  It  is  the  "neck  sweetbread" 
of  the  market.  Since  it  is  prominent  during  growth  the 
inference  is  natural  that  it  ministers  in  some  way  to  the 
process  of  development.  It  almost  vanishes  at  maturity. 
It  is  not  yet  possible  to  make  very  definite  statements 
about  the  thymus.  Removal  does  not  seriously  hinder  the 
progress  of  the  growing  animal. 

Here  and  there  in  the  body  are  firm  kernels  of  tissue, 
spoken  of  as  lymphatic  glands  or,  better,  as  lymph-nodes. 
These  may  be  regarded  as  producers  of  internal  secretion, 
though  it  is  probable  that  this*description  does  not  cover 
all  their  activities.  They  are  found  especially  in  the  neck, 
the  armpits,  the  groins,  and  in  the  mesentery.  Wherever 
placed,  each  is  set  upon  the  route  of  the  Ij^mph  as  it  comes 
from  some  region  toward  the  chest.  Thus  the  lymph-nodes 
of  the  neck  must  be  passed  by  the  lymph  that  has  had  its 
origin  in  the  head,  while  those  of  the  mesentery  intercept 
that  which  has  come  from  the  intestine.  Microscopic 
study  shows  that  the  lymph  has  to  take  a  tortuous  course 
among  the  cells  of  the  nodes.  Looked  at  from  a  mechan- 
ical standpoint  these  bodies  are  obstructions  in  the  path. 
They  are  also  suggestive  of  filters.     Their  own  cells  are 


244  NUTRITIONAL   PHYSIOLOGY 

continually  becoming  detached  and  drifting  away  in  the 
lymph,  a  phenomenon  which  is  a  tjrpe  of  internal  secre- 
tion made  visible.  They  are  believed  to  add  substances  in 
solution  as  well  as  cells  to  the  passing  fluid. 

There  is  good  ground  for  the  view  that  the  lymph- 
nodes  are  a  defense  against  the  spread  of  infection.  When 
a  boil  exists  upon  the  arm  the  nodes  above  the  place  where 
the  bacteria  are  working  so  destructively  are  usually  ob- 
served to  be  enlarged  and  tender.  The  lymph  which  is 
returning  from  the  seat  of  the  trouble  is  bearing  the  pro- 
ducts of  the  suppuration,  if  not  the  infecting  organisms 
themselves.  Apparently  this  polluted  lymph  is  more  or 
less  successfully  disinfected  before  it  is  allowed  to  pass  on 
into  the  thorax  to  merge  with  the  blood  in  the  veins.  The 
lymph-nodes  of  the  mesentery  stand  as  outworks  of  the 
bodily  fortifications  against  the  entrance  of  microbic 
invaders  from  the  intestine.  When  overpowered  in  their 
struggle  the  lymphatic  glands  themselves  become  foci  of 
infection,  as  in  the  familiar  form  of  tuberculosis  known  as 
scrofula. 

One  large  organ,  which  from  its  anatomic  relations  has 
been  called  a  ductless  gland  and  whitjh  might  be  expected 
to  have  an  internal  secretion,  has  failed  to  give  satisfac- 
tory evidence  of  such  a  function.  This  is  the  spleen,  which 
is  placed  below  the  diaphragm  to  the  left  of  the  stomach. 
It  remains  an  enigma  to  physiologists.  Its  blood-supply 
is  large  and  its  frequent  changes  of  volume,  contractions, 
and  dilations  alternating  in  a  slow  rhythm,  give  a  strong 
suggestion  of  some  well-marked  action  in  progress.  But 
it  has  never  been  shown  that  an  animal  is  affected  in  any 
characteristic  way  by  the  loss  of  the  spleen,  provided  the 
immediate  effects  of  the  severe  operation  are  survived. 
Certain  enzymes  have  been  extracted  from  the  tissue  of  the 
spleen,  which  may  have  to  do  with  the  metabolism  of  those 
peculiar  proteins  which  yield  uric  acid.  We  may  not  be 
warranted  in  asserting  that  the  spleen  has  no  useful  ser- 
vice to  perform,  but  we  can  say  that  other  organs  appear 
to  be  able  to  make  good  its  deficiency. 


CHAPTER  XXVI 
THE   NERVOUS   SYSTEM 

In  the  introductory  chapter  it  was  said  in  substance 
that  the  one  word  which  most  nearly  covers  the  work  of  the 
nervous  system  is  the  word  co-ordination.  This  state- 
ment arouses  in  one  the  impulse  to  protest  that  it  leaves 
out  of  account  the  relations  subsisting  between  the  nervous 
system  and  the  states  of  consciousness  which  are  of  the 
most  immediate  interest  to  us  all.  The  physiologist,  being 
human,  sympathizes  with  such  a  protest,  but  he  must 
continue  to  treat  his  material  for  the  most  part  from  an 
external  point  of  observation.  This  is  not  for  want  of 
respect  for  the  psychologic  method;  it  is  rather  with  frank 
recognition  of  the  vastness  of  the  realm  in  which  that 
method  is  applicable.  It  is  because  he  must  defer  to 
experts  in  that  field  that  he  will  not  enter  upon  it  as  an 
amateur. 

Most  readers  need  to  be  told  with  the  utmost  emphasis 
and  with  frequent  reiteration  that  consciousness  is  the 
accompaniment  of  an  excessively  small  share  of  the  mani- 
fold reactions  of  the  nervous  system.  In  the  words  of 
President  Hall,  it  is  "a  little  candle  burning  in  one  room  of 
the  mind's  museum."  All  that  occurs  from  first  to  last  in 
the  life  history  of  a  fish  or  a  frog  can  be  explained  as  reflex 
adjustment,  without  the  assumption  of  self-knowledge  or 
conscious  purpose  on  the  part  of  the  animal.  That  was  a 
weird  and  fascinating  picture  which  duBois  Reymond  once 
drew  of  a  world  precisely  like  our  own,  save  that  its  in- 
habitants were  unconscious.  In  such  a  world  an  artist 
without  will  or  pleasure  in  his  work  might  create  a  faultless 
statue  because  his  inherited  nervous  mechanism  and  the 

245 


246  NUTRITIONAL   PHYSIOLOGY 

existence  of  materials,  tools,  and  a  model,  made  the  result 
inevitable. 

In  the  previous  discussion  of  reflex  action  (Chapter  V) 
the  conception  was  developed  that  the  nervous  system  con- 
sists of  pathways  capable  of  transmitting  energy  in  the 
form  of  "nerve-impulses"  to  and  from  its  central  portion. 
That  central  part  is  represented  in  the  higher  forms  by 
the  brain  and  the  spinal  cord.  The  afferent  or  incoming 
paths  begin  in  localities  where  external  influences  or  stim- 
uli can  be  brought  to  bear.  The  nerve-endings  which  lie 
thus  exposed  are  called  "receptors."  They  may  be  simple 
terminal  twigs  or  bulbs  subject  to  stimulation  by  pressure 
or  temperature.  They  may  be  connected  with  elaborate 
organs  like  the  eye  and  the  ear.  The  eye  is  a  device  for 
converting  the  energy  of  certain  ether-waves  into  the  im- 
pulses that  traverse  the  optic  nerve.  The  ear  is,  at  least 
in  part,  a  device  for  transforming  the  energy  of  certain 
air-waves  into  impulses  that  run  along  the  auditory 
nerve. 

The  impulses  which  arrive  within  the  brain  or  the  cord 
may  cause  the  immediate  or  delayed  return  flow  of  im- 
pulses along  the  efferent  or  centrifugal  paths  of  the  system. 
These  lead  in  most  cases  to  the  contractile  tissues  which 
give  objective  expression  to  animal  life.  It  must  not  be 
forgotten  that  the  efferent  branches  of  the  nervous  system 
extend  also  to  various  glands  and  may  modify  secretion  as 
well  as  contraction.  It  will  be  recalled  that  efferent  im- 
pulses may  initiate  the  flow  of  tears,  of  the  saliva,  the 
gastric  juice,  and  the  adrenal  principle.  Other  instances 
are  almost  equally  clear. 

In  animals  of  all  grades  responses  of  the  reflex  type  are 
the  constant  duty  of  the  nervous  equipment.  Those  forms 
which  we  regard  as  highest  in  the  scale  are  distinguished  by 
a  second  property,  that  of  the  modification  of  the  re- 
sponses as  a  result  of  individual  experience.  By  this  is 
meant  the  capacity  for  training,  forming  habits,  or  learn- 
ing to  profit  by  the  past.  It  is  what  makes  different  mem- 
bers of  the  same  species  vary  in  their  behavior.     It  is  what 


THE    NERVOUS    SYSTEM  247 

makes  an  older  member  superior  to  a  younger  one  in  his 
powers  of  adjustment  and  maintenance.  This  is,  of  course, 
exemplified  in  the  fullest  degree  by  human  beings.  The 
prolonged  period  of  growth  and  progress  must  indicate 
a  plasticity  in  the  arrangements  of  the  nervous  elements 
that  is  hardly  ever  entirely  outlived. 

Descartes  in  the  seventeenth  century  grasped  the  two 
properties  with  his  usual  keenness.  He  pictured  the  reflex 
process  with  quaint  symbolism,  but  with  essential  correct- 
ness. He  likened  a  path  of  afferent  transmission  to  a  cord 
that  is  pulled.  A  valve  is  opened  in  the  central  organ  and 
a  fluid  released  to  flow  back  along  the  nerve.  Reaching 
a  group  of  muscles  this  fluid  w^akes  them  to  action.  Des- 
cartes had  used  different  analogies  for  the  afferent  and  the 
efferent  impulses,  making  one  a  mechanical  twitch  and  the 
other  a  spurt  of  liquid ;  we  now  believe  them  to  be  nearly 
or  quite  identical  in  character.  The  same  writer  recog- 
nized the  ability  of  the  higher  nervous  systems  to  retain 
impressions,  and  likened  the  registration  of  a  memory  to 
the  imprint  of  a  seal  upon  wax. 

Physiologically,  the  particular  mark  of  the  highly  de- 
veloped nervous  axis  is  this  capacity  for  recording  indi- 
vidual and  not  merely  racial  experience.  AnatomicaUy, 
the  advance  in  organization  is  signalized  by  an  increasing 
predominance  of  the  brain  over  the  cord  and  of  the 
' 'fore-brain"  over  the  parts  behind.  A  frog  will  carry  out 
many  complex  reactions  when  the  entire  brain  has  been 
destroyed.  That  is  to  say  that  in  the  frog  the  cord  is  ade- 
quate for  a  great  deal  of  reflex  action.  In  a  human  para- 
lytic whose  injury  has  removed  the  lower  half  of  the  spinal 
cord  from  functional  union  with  the  brain,  the  reflexes 
which  can  be  obtained  from  the  lower  extremities  are  few 
and  slight.  In  other  words,  the  cord  in  man  has  become 
less  important  as  an  organ  for  correlating  afferent  with 
efferent  impulses  and  more  important  as  the  largest  of  all 
nerves,  the  highway  to  and  from  the  brain.  We  shall  now 
set  forth  in  some  detail  certain  of  the  adaptive  changes 
which  are  constantly  occurring  through  the  mediation  of 


248  NUTKITIONAL   PHYSIOLOGY 

the  brain  quite  apart  from  attention  or  desire  on  the  part 
of  the  subject. 

This  kind  of  subconscious  control  of  organs  is  well  illus- 
trated in  the  case  of  the  respiratory  center.  About  a 
hundred  years  ago  it  was  found  by  French  workers  that 
cutting  across  the  nervous  axis  at  the  point  where  it  leaves 
the  skull — that  is,  where  the  brain  passes  into  the  cord — 
instantly  stops  the  breathing.  A  similar  cut  a  little  higher 
up  does  not  have  this  immediately  fatal  effect..  It  was  in- 
ferred that  the  part  of  the  brain  next  adjoining  the  cord 
must  contain  the  special  cells  which  stand  in  charge  of  the 
respiratory  muscles.  This  section  of  the  brain,  just  within 
the  skull,  is  called  the  medulla.  Later  experiments  have 
confirmed  the  early  belief  that  the  breathing  is  governed 
from  this  region.  The  muscles  employed  are  not  in  them- 
selves automatic.  Every  contraction  which  they  make  is 
referable  to  a  metabolic  change  in  the  cells  of  the  medulla. 
So  there  is  a  fundamental  difference  between  the  breathing 
movements  and  the  beating  of  the  heart.  The  former 
are  due  to  central  causes;  the  latter,  to  an  innate  quality 
of  the  contractile  substance. 

The  respiratory  center  is  frequently  involved  in  reflex 
action.  In  fact,  it  is  easy  to  convince  one's  self  that  hardly 
any  considerable  reflex  occurs  without  some  disturbance  of 
the  breathing  as  an  incident.  Any  sort  of  shock  will  in- 
fallibly change  the  depth,  regularity,  or  some  other  feature 
of  the  movements.  Outcries  following  such  shocks  are 
merely  respiratory  reflexes,  but  the  center  is  not  prompted 
to  each  successive  discharge  by  afferent  impulses ;  it  shows 
us  the  possibility  of  another  means  of  regulation.  This 
is  through  the  influence  upon  the  nerve-cells  of  the 
chemical  composition  of  the  blood  and  lymph  in  the 
vicinity.  When  exercise  is  taken  the  breathing  is  involun- 
tarily deepened.  The  cause  of  this  adjustment  is  found  in 
the  increase  of  carbon  dioxid  in  the  circulation.  The 
center  is  remarkably  sensitive  to  any  rise  in  the  percentage 
of  this  gas.  Conversely,  it  is  temporarily  paralyzed  by  a 
reduction  of  the  circulating  carbon  dioxid  to  an  unusually 


THE    NERVOUS    SYSTEM  249 

low  level.  Variations  of  the  oxygen  of  the  blood  affect  its 
action  surprisingly  little. 

An  important  work  of  the  nervous  system  and  one  which 
is  quite  overlooked  by  the  layman  consists  in  the  regula- 
tion of  blood-flow.  In  our  previous  description  of  the 
plan  of  the  circulation  only  its  mechanical  principles  were 
discussed :  the  heart  was  spoken  of  as  a  force-pump  and  the 
vessels  as  elastic  tubes.  This  is  a  proper  presentation  so 
far  as  it  goes,  but  we  must  now  proceed  to  show  that  the 
whole  circulatory  apparatus  is  living,  and  being  so  has  the 
biologic  characteristic  of  adaptation.  The  changes  by 
which  the  ever-varying  requirements  of  the  body  are  met 
are  of  two  kinds,  changes  in  the  force  and  frequency  of  the 
heart-beat  and  changes  in  the  caliber  of  the  small  arteries 
and  veins.  The  former  are  brought  about  by  the  cardiac 
nerves;  the  latter,  by  a  department  of  the  nervous  mechan- 
ism which  we  call  the  vasomotor  system. 

It  is  a  matter  of  familiar  experience  that  the  heart-rate 
is  subject  to  striking  variations.  From  an  average  of  per- 
haps 65  per  minute,  when  one  is  lying  in  comfortable  re- 
laxation, it  can  be  driven  to  150  or  more,  when  one  is 
hurr>4ng  up  a  grade.  Such  a  change  under  the  influence  of 
muscular  activity  is  evidently  purposeful.  The  working 
tissues  need  a  s^dfter  current  of  blood  to  supply  them  with 
oxygen  and  to  bear  away  their  waste.  The  lungs  must  be 
visited  at  shorter  intervals  to  keep  normal  the  gaseous  com- 
position of  the  blood.  Muscular  activity  is  not  to  be  sup- 
ported by  increased  breathing  alone;  it  is  equally  necessary 
that  there  shall  be  acceleration  of  the  circulation.  As  the 
quick,  deep  breathing  of  one  who  is  taking  exercise  be- 
speaks a  governing  center  for  the  muscles  employed,  so 
the  rapid  beating  of  his  heart  suggests  a  central  control  of 
that  organ. 

Experimental  proof  of  the  central  regulation  of  the  heart 
is  ample  and  detailed.  Branches  of  the  nervous  system 
reach  it  from  two  distinct  sources  and  are  contrasted  in 
their  effect  upon  it.  One  set  of  fibers  is  said  to  be  inhib- 
itory, the  other  to  have  an  accelerator  action.     The  terms 


250  NUTRITIONAL   PHYSIOLOGY 

would  seem  to  explain  themselves.  The  inhibitory  fibers 
restrain  the  heart  much  of  the  time  from  beating  at  the 
rate  which  it  would  exhibit  if  nervous  regulation  were 
entirely  withdrawn.  An  exaggerated  inhibitory  influence 
may  weaken  its  working  to  the  point  where  the  circulation 
becomes  quite  inadequate  and  faintness  is  produced.  In 
laboratory  trials  actual  arrest  of  the  heart  of  a  dog  may  be 
caused,  though  the  beat  is  always  resumed  within  a  minute, 
so  that  death  cannot  be  made  to  result.  We  ought  not  to 
lay  much  stress  upon  these  extreme  possibilities.  It 
would  be  absurd  to  say  that  the  function  of  the  inhibitory 
cardiac  nerves  is  to  stop  the  heart;  that  could  never  be  for 
the  best  good  of  the  animal.  We  can  say  with  more  reason 
that  their  service  is  to  economize  the  strength  of  the  heart 
and  to  provide  a  reserve  for  emergencies.  This  has  been 
described  as  a  ^ 'brake  action." 

The  accelerator  nerves  of  the  heart  have  a  stimulating 
effect  which  extends  to  the  vigor  as  well  as  to  the  rate  of 
its  beating.  When  we  observe  a  specific  increase  of  heart 
action  we  can  hardly  say  whether  it  has  been  produced  by 
positive  accelerator  influence  or  by  the  abatement  of  the 
habitual  inhibitory  control.  It  may,  of  course,  represent 
the  combined  result  of  both.  The  government  of  organs 
by  means  of  the  balanced  action  of  two  opposing  sets  of 
nerves  is  repeatedly  met  with  in  the  body.  It  can  be  de- 
monstrated for  the  stomach  and  for  other  sections  of  the 
alimentary  canal.  A  singular  example  is  afforded  by  the 
nervous  relations  of  the  iris,  the  colored  ring  surrounding 
the  black  pupil  of  the  eye.  Stimulation  of  one  nerve  causes 
contraction  of  the  pupil,  while  widening  or  dilatation  fol- 
lows the  application  of  stimuli  to  an  entirely  different 
strand  of  fibers. 

The  heart  may  quicken  the  circulation  by  beating  at  a 
more  rapid  rate  and  with  increased  power,  but  it  cannot 
send  the  blood  to  a  particular  part  of  the  body  at  the  ex- 
pense of  another  part.  The  total  blood-flow  thus  depends 
upon  the  heart  acting  under  the  balanced  sway  of  inhib- 
itory and  accelerator  nerves,  but  the  distribution  of  blood, 


THE    NERVOUS    SYSTEM  251 

the  favoring  of  organs  which  need  it,  is  the  work  of  the 
vasomotor  system.  The  walls  of  the  microscopic  vessels, 
those  which  adjoin  the  capillaries  in  particular,  are  provided 
with  muscular  elements  of  the  same  general  order  as  those 
which  produce  the  movements  of  the  digestive  tract. 
These  contractile  elements  are  connected  with  the  terminal 
branches  of  certain  nerves.  Hence  the  anatomic  study  of 
the  tissues  involved,  even  when  unsupported  by  physio- 
logic experiments,  makes  clear  the  possibility  that  the 
centers  when  acting  reflexly  or  otherwise  may  influence 
the  diameter  of  the  blood-vessels  and  the  volume  of  the 
circulation  in  any  or  all  regions  of  the  body. 

Physiology  reinforces  anatomy  at  this  point.  For  sixty 
years  it  has  been  certainly  known  that  nervous  regulation 
of  the  blood-flow  is  a  fact.  Much  earlier  than  this  the 
power  was  assumed  to  exist,  "Ubi  stimulus,  ibi  affluxus," 
they  said,  meaning  that  where  there  is  activity  there  is  an 
increase  of  blood.  The  paling  and  the  flushing  of  the  skin, 
occurring  either  in  response  to  external  changes  of  tempera- 
ture or  emotional  conditions,  are  strongly  suggestive  of  a 
central  command  of  the  vessels.  This  has  been  taken  for 
granted  in  our  own  treatment  of  the  matter  of  the  main- 
tenance of  a  normal  body  temperature.  Much  as  the  heart 
is  reached  by  impulses  which  inhibit  it  and  by  others  which 
spur  it  to  a  greater  expenditure  of  energy,  the  small  arteries 
and  veins  are  subject  to  antagonistic  influences.  We  speak 
of  a  vasoconstrictor  effect  when  we  mean  a  reinforcement 
of  the  existing  tone,  and  we  use  the  word  vasodilator  in 
the  opposite  sense,  that  is,  to  characterize  changes  in 
which  the  degree  of  contraction  is  lessened,  with  the  result 
that  the  blood  finds  its  way  in  greater  quantity  through  the 
affected  vessels. 

The  difference  between  the  cardiac  and  the  vasomotor 
factors  in  the  control  of  the  circulation  can  be  made  plain 
by  means  of  an  illustration  borrowed  from  the  laundry. 
Suppose  that  a  supply-pipe  runs  along  above  a  row  of 
tubs  and  bears  a  faucet  for  each.  Suppose,  also,  that  there 
is  a  stop-cock  on  the  pipe  before  it  reaches  the  tubs.      Fi- 


252 


NUTRITIONAL   PHYSIOLOGY 


nally,  to  make  the  correspondence  closer,  assume  that  the 
faucets  can  never  be  shut  tight  (for  it  is  not  Hkely  that  the 
blood-vessels  can  be).  Now,  by  manipulating  the  cock 
(H)  we  can  increase  or  diminish  the  outflow  into  all  the 
tubs,  but  we  cannot  in  this  way  hasten  the  filling  of  one 
more  than  another.  This  is  obviously  the  type  of  cardiac 
regulation  in  the  living  system.     The  heart  is  appealed  to 


Fig.  23. — The  supply  to  all  the  tubs  is  controlled  at  the  "  shut 
ofif  "  (H),  while  the  share  flowing  into  each  compartment  is  regulated 
by  its  own  faucet  (F). 


when  there  is  a  general  and  widespread  demand  for  more 
blood  per  unit  of  time,  muscular  activity  being  by  far  the 
most  important  occasion. 

The  faucets  over  the  several  tubs  are  the  symbols  of 
vasomotor  equipment.  By  using  them  we  can  change  the 
share  of  the  total  stream  which  shall  enter  any  compart- 
ment. If  our  only  desire  is  to  fill  one  tub,  we  can  open 
widely  that  faucet  and  close  the  others  as  far  as  possible. 
The  vasomotor  system  actually  operates  in  that  way  to  the 


THE    NERVOUS    SYSTEM  253 

extent  that  a  dilation  in  one  large  area  seems  often  to  be 
offset  by  a  compensatory  constriction  in  another.  We 
count  on  this  reciprocal  relation  in  our  medical  and  hy- 
gienic practice.  For  instance,  we  think  that  if  we  can  en- 
courage blood-flow  at  the  surface  we  shall  reduce  it  in  the 
deeper  parts.  This  is  often  an  object,  since  congestions 
are  important  features  of  many  disorders.  We  antici- 
pate this  balanced  reaction  when  we  make  use  of  hot  ap- 
plications and  when  we  give  alcohol  for  a  cold. 

No  one  is  likely  to  overestimate  the  services  rendered 
by  the  vasomotor  system.  It  has  both  a  general  and  a 
local  action.  In  the  first  sense  it  maintains  a  certain 
average  state  of  contration  in  the  arteries — the  arterial 
tone — which  keeps  up  to  a  desirable  level  the  pressure  in' 
the  arterial  trunks.  It  is  this  pressure  which  guarantees 
the  prompt  and  sufficient  supply  of  blood  wherever  the 
paths  are  opened.  The  principle  is  the  same  that  is  ob- 
served in  the  mains  distributing  water  through  the  streets 
of  a  city.  The  pressure  must  be  great  enough  to  keep  up 
the  supply  on  the  high  ground  as  well  as  the  low.  Leakage 
or  wasteful  use  of  water  will  lower  the  pressure  and  the 
failure  will  be  noted  first  on  the  hills.  An  unusual  dila- 
tion of  many  blood-vessels,  such  as  probably  accompan- 
ies an  attack  of  indigestion  where  the  abdominal  organs  are 
engorged,  may  lessen  the  pressure  and  the  volume  of  blood 
passing  through  the  brain  wuth  the  consequence  that  there 
is  faintness.  Partial  relief  is  found  in  lying  dowTi  because 
the  factor  of  gravity  is  eliminated  and  the  head  given  an 
equal  chance  with  the  rest  of  the  body. 

We  notice  the  failure  of  the  vasomotor  system  to  do  its 
duty  in  cases  like  the  above.  We  ought  also  to  appreciate 
how  remarkable  it  is  that  the  adjustments  usually  occur 
with  such  smoothness  and  success.  It  is  really  a  wonder- 
ful thing  that  we  can  rear  up  the  elongated  human  form 
from  a  horizontal  to  a  vertical  position  without  entirely 
deranging  the  circulatory  mechanism.  That  the  blood 
does  not  distend  the  vessels  below  the  heart  and  forsake 
those  above  must  be  due  in  a  great  measure  to  vasomotor 


254  NUTRITIONAL   PHYSIOLOGY 

correction.  Doubtless,  when  one  rises  to  his  feet  in  the 
morning  one  is  saved  from  falling  in  a  faint  by  the  prompt- 
ness with  which  the  arterial  tone  in  the  lower  extremities 
is  raised  by  the  nervous  system  to  fight  back  the  threat- 
ened excess  of  blood.  It  is  easy  to  see  what  would  happen 
in  a  lifeless  model  under  similar  conditions. 

While  our  vasomotor  reactions  are  usually  executed 
without  our  conscious  attention,  it  has  always  to  be 
borne  in  mind  that  close  ties  exist  between  the  higher  and 
the  lower  regions  of  the  nervous  fabric.  Emotions  register 
themselves  in  changes  of  blood-flow.  The  obedience  of 
the  vasomotor  center  to  hypnotic  suggestion  is  almost 
unlimited.  It  would  appear  that  this  division  of  the  ner- 
vous system  is  the  chief  mediator  in  the  correction  of  the 
many  ills  which  unquestionably  yield  to  Christian  Science 
and  kindred  ministrations.  A  normal  circulation  goes 
far  to  insure  normal  nutrition  and  irritability  in  each  part, 
and  these  states  are  the  fundamentals  of  health.  Fur- 
thermore, the  feeling  of  health — ^which  is  not  always  the 
same  thing  as  the  possession  of  health — ^must  depend,  in 
the  absence  of  more  positive  sensations,  on  the  character 
of  the  cerebral  circulation.  Assuming  the  blood  to  be 
chemically  normal  a  moderate  change  in  the  amount 
passing  through  the  brain  may  make  all  the  difference 
between  the  sense  of  power  and  the  feeling  of  utter  help- 
lessness. 

Beside  dictating  the  character  of  the  breathing  and 
determining  the  volume  and  distribution  of  the  blood- 
flow,  the  lower  part  of  the  brain  exercises  control  of  the 
alimentary  canal  and,  in  varying  degree,  of  the  glands. 
To  m_ention  these  facts  is  merely  to  recapitulate  what  has 
already  been  pointed  out.  It  must  be  recalled  that  the 
government  of  the  glands  is  partly  through  direct  com- 
munication mth  their  secreting  cells  and  partly  through 
vasomotor  regulation  of  their  blood-supply.  In  many 
cases  of  sustained  glandular  activity  both  features  exist 
at  the  same  time. 


CHAPTER  XXVII 

THE  NERVOUS   SYSTEM— ITS   HIGHER  WORK 

"  Our  wills  are  ours,  we  know  not  how.  ..." 

In  the  last  chapter  emphasis  was  constantly  placed  on 
the  subconscious  nature  of  the  multifarious  adjustments 
which  are  each  moment  secured  through  the  action  of  the 
central  nervous  system.  We  are  accustomed  to  think 
that  the  use  of  our  sense-organs  and  the  emplojmaent  of 
our  skeletal  muscles  are  activities  with  which  consciousness 
is  far  more  closely  concerned.  While  this  is  broadly 
true,  we  need  to  recognize  that  a  great  part  of  these  re- 
actions also  goes  on  without  our  notice  and  beyond  the 
reach  of  our  intervention.  This  is  readily  admitted  of  the 
breathing.  It  will  be  found  almost  equally  characteristic 
of  the  maintenance  of  the  balance  at  rest  and  during 
locomotion. 

The  ability  to  stand  is  dependent  on  the  occurrence  of 
inconspicuous  but  indispensable  reflexes  which  check  each 
swaying  movement  of  the  body  as  it  threatens  a  fall. 
When  one  walks  the  attention  seems  for  the  most  part  to 
be  detached  from  the  elaborate  muscular  performance  and 
to  be  given  to  other  matters.  The  contact  of  the  feet 
with  the  ground,  the  gliding  of  one  joint  surface  on  another, 
the  shifting  of  stresses  from  one  muscle  or  tendon  to  a 
neighbor — these  local  changes  become  in  a  regular  se- 
quence the  source  of  impulses  which  ascend  to  the  brain 
and  evoke  appropriate  responses.  It  has  been  generally 
believed  that  the  division  known  as  the  cerebellum  has  a 
peculiar  importance  in  this  connection. 

The  position  of  the  thinker  himself  with  reference  to  the 
great  afferent  and  efferent  departments  of  his  nervous 

255 


256  NUTRITIONAL   PHTSIOLOGY 

system  may  be  likened  to  that  of  the  general  in  command 
of  a  great  army.  From  his  headquarters  he  can  see  but 
a  small  proportion  of  his  troops.  Their  line  of  battle 
stretches  for  miles  beyond  his  sight.  He  issues  the  order 
for  a  general  advaaee.  His  aides  ride  to  the  right  and  left, 
bearing  the  word  to  the  commanders  of  corps  and  divisions. 
The  simple  act  of  the  general  has  been  followed  by  a  train 
of  events  which  becomes  each  moment  more  difficult  to 
trace.  The  original  order  is  transmitted  to  officers  of 
lower  rank  and  interpreted  by  them  in  conformitj^  with 
local  needs.  ^Tien  the  private  soldiers  are  at  last  put  m 
motion  it  is  at  the  word  of  colonels  and  captaias.  The 
impossibihty  of  ha'^ing  the  voice  of  the  commaiider-in- 
chief  the  source  of  guidance  in  each  companj^  is  perfectly 
e^ddent.  It  is  not  merely  that  he  is  too  far  removed  from 
most  of  his  men,  but  that  there  are  too  many  problems 
arisiag  at  one  time.  The  minor  ones  must  be  solved  at 
the  discretion  of  his  subordinates. 

So  in  the  h-vdng  body  the  wdll  to  walk — ^which  seems  as 
simple  as  the  dictation  of  the  first  order  from  headquarters 
— ^is  promptly  followed  by  the  action  of  many  ner^^ous 
mechanisms  whose  function  is  to  distribute  impulses  and  to 
apply  them  in  helpful  sequence.  We  have  no  sense  of  the 
subch^dsion  which  is  involved.  We  cannot  analyze  the 
groups  of  muscular  movements  which  take  place.  Yet 
it  is  plain  that  there  is  an  apportionment  of  stimulation, 
more  to  this  muscle  and  less  to  that,  T^dthout  which  the 
effective  result  could  not  be  secured.  How  utterly  we 
should  fail  in  the  attempt  to  regulate  the  part  taken  by 
each  of  a  hundred  co-operating  muscles  by  giving  attention 
to  each  in  its  turn!  The  efforts  necessitated  would  be  like 
those  of  the  general  deprived  of  his  staff  and  messengers 
wiio  should  seek  to  ride  swiftly  from  point  to  point  in  the 
endeavor  to  direct  all  his  soldiers  by  his  spoken  word. 
The  organization  of  the  highly  developed  nervous  system 
is  that  of  a  disciplined  hierarchy. 

The  comparison  we  have  been  using  can  be  made  to 
serve  even  further.      We  can  find  in  it  a  place  for  the  af- 


THE    NERVOUS  SYSTEM — ITS    HIGHER   WORK        257 

ferent  side  of  nervous  action.  We  have  said  that  reflex 
elements  can  be  found  in  almost  any  elaborate  movement. 
In  walking,  for  example,  the  assumption  of  a  given 
position  by  the  body  insures  the  return  to  the  centers  of 
impulses  which  cause  the  next  appropriate  change.  We 
do  not  consciously  resolve  to  take  each  step.  The  very 
idea  is  painful.  One  step  accomplished  makes  the  taking 
of  another  the  natural  sequel.  Turning  to  our  analogy, 
we  readily  see  that  when  the  grand  advance  is  under  way 
the  continuance  of  the  march  will  probably  be  intrusted 
to  the  direction  of  lower  officers.  Difficulties  encountered 
they  will  report  to  the  general  if  this  seems  necessary; 
minor  obstacles  they  will  meet  upon  their  own  responsi- 
bility. Thus  when  we  walk  we  may  be  aware  of  gutters 
to  be  crossed,  but  we  do  not  notice  at  all  the  incessant 
slight  adjustments  which  are  made  for  the  lesser  inequal- 
ities of  the  path. 

The  human  brain  contains  at  birth  connections  which 
make  possible  the  execution  of  a  moderate  number  of 
useful  reflexes.  Among  these  are  sucking,  winking,  cough- 
ing, sneezing,  and  vomiting.  Of  course,  there  are  also  the 
requisite  mechanisms  for  the  government  of  breathing, 
the  circulation,  and  the  digestive  tract.  Such  a  brain 
appears  to  differ  from  that  of  one  of  the  lower  animals 
chiefly  in  its  capacity  for  continued  development.  Loeb 
has  shown  that  all  that  is  acquired  in  the  experiences  which 
come  to  the  child  may  be  covered  by  the  term  "associative 
memory."  The  expression  as  used  by  him  does  not  refer 
to  the  power  to  review  past  experience  as  a  conscious 
process,  but  only  to  the  power  to  acquire  new  reactions  to 
environmental  conditions.  An  early  gain  in  this  direction 
is  the  attainment  of  the  ability  to  reach  after  and  grasp  an 
object  which  has  stimulated  the  \dsual  department  of  the 
nervous  system.  This  seems  to  signify  the  opening  of  a 
pathway  in  the  brain  from  the  region  receiving  impulses 
from  the  retina  to  the  region  from  which  impulses  go  out  to 
the  muscles  controlling  the  hand. 

In  similar  fashion  we  can  picture  the  acquirement  of  one 
17 


258  NUTRITIONAL   PHYSIOLOGY 

accomplishment  after  another.  All  imitative  action  must 
be  made  possible  by  the  establishment  of  bonds  between 
receiving  and  discharging  stations  in  the  surface  gray 
matter  of  the  cerebrum.  As  these  connections  are  formed 
the  conduct  of  the  individual  under  given  circumstances 
becomes  predictable;  in  other  words,  we  have  here  the 
physical  basis  of  mannerisms  and  habits.  It  is  even  per- 
missible to  say  that  the  foundations  of  character  are  defined 
in  the  direction  which  these  association  ties  are  found  to 
take.  All  of  education,  so  long  as  it  is  viewed  from  with- 
out and  not  from  within,  consists  in  the  cultivation  of 
response  to  varied  influences.  There  can  be  no  doubt  that 
the  anatomic  accompaniment  consists  in  the  multipli- 
cation of  brain  pathways. 

During  the  last  hundred  years  the  question  has  been 
much  debated  whether  particular  powers  have  definitely 
localized  registration  in  the  structure  of  the  brain.  In  the 
first  half  of  the  nineteenth  century  the  phrenologists  at- 
tracted much  favorable  attention  by  their  claims  of  very 
precise  subdivision  of  the  brain  into  '^organs  of  different 
faculties."  The  notion  of  ''bumps"  is  still  current,  though 
it  is  usually  referred  to  in  a  jocular  spirit.  The  literature 
of  phrenology  is  very  large,  and  it  is  probable  that  it  would 
repay  a  critical  review,  but  the  advocates  of  the  cult  went 
much  farther  than  the  facts  warranted  and  their  concep- 
tions fell  into  disrepute.  By  1850  the  reaction  had  carried 
scientific  opinion  to  the  belief  that  there  is  little  localiza- 
tion in  the  brain. 

Since  then  a  great  many  experimental  studies,  together 
with  observations  of  the  consequences  of  injuries  suffered 
by  human  brains,  have  united  to  encourage  the  view  that 
there  is  some  degree  of  localization.  The  new  doctrines 
have  not  followed  the  old  traditions  at  all.  Where  the 
phrenologists  sought  to  refer  mental  functions  to  particular 
regions  of  the  brain  surface,  modern  students  have  looked 
with  more  success  for  the  central  representation  of  bodily 
processes.  They  have  shown  that  a  certain  area  stands  in 
definite  relation  with  the  skeletal  muscles,  that  another 


THE    NERVOUS   SYSTEM — ITS    HIGHER    WORK        259 

area  is  the  place  for  the  reception  of  the  visual  impulses, 
and  so  on.  In  most  respects  the  human  brain  has  been 
found  to  be  organized  in  a  manner  closely  corresponding 
with  what  is  traceable  in  the  brain  of  the  ape  and  hinted 
at  in  the  brain  of  the  dog. 

Yet  the  distinctive  accomplishments  in  which  man  ex- 
cels the  lower  animals  must  be  coupled  in  some  way  with 
his  cerebral  equipment.  One  or  two  of  these  distinguishing 
features  seem  to  have  quite  definite  positions.  This  is 
especially  clear  in  the  case  of  the  well-recognized  ''speech 
center."  The  belief  is  commonly  held  that  the  mechanisms 
which  are  correlated  with  such  acquired  powers  as  speech, 
reading,  and  writing  are  restricted  to  one-half  of  the 
brain,  usually  the  left.  The  right  hand,  which  in  most 
subjects  is  so  much  superior  in  skill,  is  governed  from  the 
left  side  of  the  cerebrum.  A  charming  account  of  the 
apparent  relations  between  the  human  brain  and  human 
capacities  is  given  in  W.  H.  Thomson's  ''Brain  and  Per- 
sonality." The  claims  made  for  precise  localization  in 
that  book  are  regarded  by  many  conservative  writers  as 
extreme.  It  is  certainly  fair  to  say  that  at  present  the 
most  learned  interpreters  of  brain  physiology  are  placing 
emphasis  on  the  development  of  paths  between  centers 
rather  than  on  the  organization  of  the  centers  themselves. 

The  characteristic  of  early  life  is  the  ease  and  freedom 
with  which  these  paths  are  opened  and  the  comparative 
frequency  with  which  they  are  changed.  During  the  long 
period  of  mature  efficiency  they  are  less  subject  to  multi- 
plication and  are  used  with  increasing  regularity  and  with 
rarer  deviations.  The  man  is  becoming  a  creature  of 
haVjit  and  acquiring  "ruts."  In  old  age,  as  has  been  finely 
said,  the  nervous  system,  instead  of  holding  a  prophecy 
of  what  may  be,  contains  a  record  of  the  past.  It  is  a 
fascinating  fancy — though  it  is  nothing  more — that  a 
physician  of  surpassing  insight  might  look  upon  the  warp 
and  woof  of  fibers  in  a  dead  brain  and  tell  us  of  the  tastes, 
talents,  and  pursuits  of  its  former  possessor.  When  we 
walk  through  the  rooms  of  a  deserted  house  we  can  tell 


260  NUTRITIONAL   PHYSIOLOGY 

by  the  worn  places  on  the  floors  and  thresholds  and  by  the 
grimy  edges  of  the  doors  just  where  the  tenants  came  and 
went  most  often.  The  lifeless  brain  must  bear  a  more 
subtle  registry  of  the  same  order. 

Afferent  fibers  reach  the  brain  from  all  parts  of  the  body. 
Many  of  these  have  had  their  origin  within  its  tissues, 
where  they  are  normally  stimulated  by  conditions  that 
belong  to  the  organism  itself  rather  than  to  its  environ- 
ment. The  impulses  that  enter  the  brain  from  such 
sources  are  most  of  the  time  serving  their  purpose  in  pro- 
moting subconscious  adaptive  reflexes.  When  they  affect 
consciousness  it  is  to  bring  to  the  attention  the  so-called 
''general  sensations" — those  feelings  which  we  refer  to 
states  of  the  organs.  Such  are  hunger  and  thirst,  many 
kinds  of  pain,  satiety,  nausea,  faintness,  fatigue,  and  the 
like.  The  majority  of  these  general  sensations  seem  to 
signify  conditions  that  need  to  be  rectified  and  they  are 
mostly  unpleasant. 

Contrasted  with  these  are  the  "special  sensations," 
which  are  referred  to  causes  acting  upon  the  afferent  ap- 
paratus from  outside.  Various  terminal  structures  are 
developed  at  or  near  the  surface  of  the  body  which  serve 
to  transmute  different  forms  of  environmental  energy 
into  nerve-impulses.  Such  structures  are  called  sense 
organs  or  ''receptors."  A  very  suggestive  distinction  has 
been  made  between  the  "proprio-receptors,"  which  are 
affected  only  by  the  literal  contact  of  the  stimulating  sub- 
stance, and  the  "distance-receptors,"  which  respond  to 
forms  of  energy  radiating  from  places  more  or  less  remote. 
The  nerve-endings  in  the  skin  which  are  acted  upon  by 
pressure  and  by  temperature  changes  are  proprio-recep- 
tors.  So  are  those  in  the  tongue  on  which  various  dis- 
solved substances  take  effect,  giving  the  conscious  sub- 
ject sensations  of  taste.  The  olfactory  endings  high  up  in 
the  nasal  cavities  are  reckoned  by  most  writers  to  be  in  the 
same  class. 

The  ear  is  a  distance  receptor.  The  energy  which  sets 
its  intricate  mechanism  into  vibration  may  have  originated 
as  far  away  as  the  thunder-cloud  hanging  near  the  horizon. 


THE    NERVOUS    SYSTEM — ITS    HIGHER   WORK        261 

The  possession  of  an  ear  greatly  extends  the  compass  of  the 
environment  which  can  exert  directing  influences  on  the 
conduct  of  an  animal.  If  this  is  true  of  the  ear,  how  shall 
we  estimate  the  widening  of  environment  that  comes 
with  the  addition  to  the  receptor  system  of  an  eye!  Our 
ability  to  see  the  stars  means  nothing  less  than  this:  that 
the  reactions  of  the  organism  so  endowed  may  be  modified 
by  energy  proceeding  from  the  incomprehensible  distances 
of  the  stellar  universe. 

Throughout  this  book  we  have  held  steadily  to  the  point 
of  view  defined  in  its  very  first  paragraphs :  that  all  living 
things  are  transformers  of  energy,  and  engaged  so  long  as 
they  live  in  reacting  according  to  the  principles  of  mechan- 
ics and  chemistry  in  response  to  external  changes.  A  pres- 
entation in  this  spirit  provokes  resentment  and  protest 
from  many  readers.  It  seems  to  leave  out  of  account  all 
that  is  instinctively  held  to  be  highest  and  finest  in  human 
life.  To  this  remonstrance  we  are  glad  to  give  place. 
The  scientist  is,  after  all,  a  man,  and  no  scientist  was  ever 
so  ruthlessly  logical  as  to  convince  himself  that  his  friend 
was  no  more  than  a  reflex  mechanism.  The  impression 
that  the  study  of  science  deprives  one  of  the  philosophic 
outlook  and  of  the  conviction  of  moral  responsibility 
belongs  to  the  earlier  stages  of  the  student's  experience. 
Later  it  is  seen  that  no  incentive  to  right  conduct  and  no 
worthy  consolation  is  to  be  taken  away. 

A  few  years  ago  a  brilliant  astronomer  was  concluding 
a  course  of  lectures  in  which  he  had  traced  the  long  story 
of  planetary  evolution.  He  had  pictured  the  ages  of  forma- 
tive process,  the  slow  condensation  and  cooling  of  the 
globe,  the  gradual  approach  to  conditions  suitable  for  or- 
ganic life.  He  had  sketched  the  brief  flourishing  of  that 
life,  the  remorseless  chilling  of  the  planet,  and  its  frigid 
and  sterile  old  age.  His  hearers  were  weighed  down  with 
the  appalling  sense  of  futility  and  insignificance.  At  the 
very  last  he  asked  abruptly:  Which,  after  all,  is  the  greater 
— these  awful  ranges  of  time  and  reaches  of  space  or  the 
mind  of  man  which  comprehends  and  ponders  them? 


262  NUTRITIONAL   PHYSIOLOGY 


REFERENCES 

Chapter  I:  Sedgwick  and  Wilson,  "  General  Biology,"  Holt,  New 
York,  1895,  chaps,  i,  ii,  iii. — Bunge,  "  Text-book  of  Physio- 
logical and  Pathological  Chemistry,"  Kegan  Paul,  Trench, 
Triibner  &  Co.,  London,  1890,  chap.  i. 

Chapter  II:    Sedgwick  and  Wilson,  chap.  xvii. — Bunge,  chaps,  ii,  iii. 

Chapter  III:  Howell,  ''A  Text-book  of  Physiology,"  4th  ed., 
Saunders,  Philadelphia,  1911,  chap.  xl. — Fischer,  "  Physiology 
of  Alimentation,"  Wiley,  New  York,  1907,  chap.  iv. — Starling, 
"  Recent  Advances  in  the  Physiology  of  Digestion,"  Keener, 
Chicago,  1906,  chaps,  i,  ii. — Bayliss,  ''  The  Nature  of  Enzyme 
Action,"  Longmans,  London,  New  York,  Bombay,  and  Cal- 
cutta, 1908. 

Chapter  IV:  Hough  and  Sedgwick,  "The  Human  Mechanism," 
Ginn,  Boston,  1906,  chaps,  iii,  iv. 

Chapter  V:    Hough  and  Sedgwick,  chap.  vii. 

Chapter  VI:  Kimber,  "Anatomy  and  Physiology  for  Nurses," 
3d  ed.,  Macmillan,  New  York,  1910,  chap.  xiv. 

Chapter  VII:  Howell,  chap.  xli. — Fischer,  chaps,  i,  iii,  v,  x. — Starling, 
chap.  iii. 

Chapter  VIII:  Howell,  chap,  xxxix. — Cannon,  "The  Mechanical 
Factors  of  Digestion,"  Longmans,  New  York,  1911,  chaps,  iv, 
V,  vi,  ix,  X. — ^For  Alexis  St.  Martin,  see  Osier,  "  An  Alabama 
Student  and  Other  Biographical  Essays,"  Oxford  University 
Press,  American  Branch,  New  York,  1909,  pp.  159-188. 

Chapter  IX:  Howell,  chap.  xlii. — Starling,  chap.  iv. — Fischer, 
chaps.  V,  vi,  xi. 

Chapter  X:  Howell,  chap,  xliii. — Cannon,  chap.  xi. — Starling, 
chap.  V. — Fischer,  chaps,  vi,  viii,  xii,  xiii. 

Chapter  XI:    Cannon,  chap.  xii. — Howell,  chap,  xliii. 

Chapter  XII:  Martin,  "  The  Human  Body,"  9th  ed.,  Holt,  New 
York,  1910. 

Chapter  XVII:     Howell,  chap.  xxiv. 

Chapter  XIII:    Martin,  chaps,  xix  to  xxi. 

Chapter  XIV:  Howell,  chaps,  xlii,  xliii. — Fischer,  chaps,  xiv  to  xvii. 
—Starling,  "  The  Fluids  of  the  Body,"  Keener,  Chicago,  1909, 
chaps,  ii,  iii. 

Chapter  XV:  Howell,  chap,  xlviii. — Lusk,  "  Elements  of  the  Science 
of  Nutrition,"  2d  ed.,  Saunders,  Philadelphia,  1909,  chap.  vii. 

Chapter  XVI:  Howell,  chap,  xlvii. — Lusk,  chaps,  iv,  v. — Abder- 
halden,  "  Text-book  of  Physiological  Chemistry,"  Wiley, 
New  York,  1908,  chaps,  x  to  xiv. 

Chapter  XVII:  Howell,  chaps,  xxxvi,  xiv. — Martin,  chaps,  xxiii, 
xxiv,  xxxi. 

Chapter  XVIII:  Howell,  chap,  xlvii. — Lusk,  chap.  i. — Schafer, 
"  Text-book  of  Physiology,"  Macmillan,  New  York,  1898,  vol. 
i,  article  "  Metabolism,"  p.  868.— Tiegerstedt,  "  A  Text-book 
of  Human  Physiology,"  Appleton,  New  York,  1906,  chap.  iv. 

Chapter  XIX:  Howell,  chaps.  1,  U. — Lusk,  chap.  i. — For  Count 
Rumford,  see  Jordan,  "  American  Men  of  Science,"  Holt, 
New  York,  1910. 


THE    NERVOUS    SYSTEM— ITS    HIGHER   WORK        263 

Chapter  XX:    Howell,  chaps.  1,  li, — Lusk,  many  sections,  especially 
chaps,  vi,  viii. 

Chapter  XXI:    Hough  and  Sedgwdck,  chap.  xii. — Lusk,  chap.  iii. 

Chapters  XXII,  XXIII:  Hough  and  Sedgwick,  chap.  xix. — Lusk, 
chap.  ix. — Chittenden,  "  Physiological  Economy  in  Nutrition  '' 
Stokes,  New  York,  1904.— Fletcher,  ''  The  A-B-Z  of  our  own 
Nutrition,"  A,  B,  C  Life  Series,  New  York,  1903.— Benedict 
"  American  Journal  of  Physiology,"  August,  1906.— Crichton- 
Browne,  ''  Journal  of  the  Royal  Institute  of  Public  Health  " 
1908,  vol.  xvi,  pp.  471,  527.— Meltzer,  "  The  Factors  of  Safety 
in  the  Animal  Structure  and  Animal  Economy,"  Jour,  of  Amer 
Med.  Assoc,  1907,  vol.  xlviii,  p.  655.— Pyle,  "  Personal  Hy- 
giene," 4th  ed.,  Saunders,  Philadelphia,  1910,  pp.  9  to  51.— 
Herter,  "  Common  Bacterial  Infections  of  the  Digestive  Tract," 
etc.,  Macmillan,  New  York,  1907.— Metchnikoff,  "  The  Nature 
of  Man,"  Putnam,  New  York,   1903. 

Chapter  XXIV:     Billings,   "Physiological  Aspects  of  the  Liquor 
Problem,"  Houghton,  Mifflin  &  Co.,  Boston,  1903.— Kelynack 
"  The  Drmk  Problem,"  etc.,  Methuen  &  Co.,  London,  1909. 
— Hough  and  Sedgwick,   chap.   xx. 

Chapter  XXV:   Howell,  chap.  xlvi. — Tigerstedt,  chap.  xi. 

Chapters  XXVI,  XXVII:  Tigerstedt,  chaps,  xxii  to  xxiv.— Howell, 
chaps,  ix,  X,  xiii.— Thomson,  "  Bram  and  Personality,"  Dodd, 
Mead  &  Co.,  New  York,  1907.— Munsterberg,  "Psycho- 
therapy," Moffat,  Yard  &  Co.,  New  York,  1909,  chap.  iii. 
—James,  "  Psychology,"  Holt  &  Co.,  New  York,  1892.— Pyle, 
pp.  275  to  314.  ^    ' 

Additional  Reference  Books  on  Foods  and  Nutrition 

Hutchison,  "  Food  and  Dietetics,"  Wood,  London,  1911. 
Thompson,  "  Practical  Dietetics,"  Appleton,  New  York,  1902 
Pattee,  "  Practical  Dietetics,"  6th  ed.,  published  by  the  author 

Mount  Vernon,  New  York,  1910. 
Locke,  "  Food  Values,"  Appleton,  New  York,  1911. 
Sadler,  "  The  Science  of  Living,"  McClurg,  Chicago,  1910. 
Sherman,  "  Chemistry  of  Food  and  Nutrition,"  Macmillan,  New 

York,  1910. 
Jordan,  "  Principles  of  Nutrition,"  Macmillan,  New  York,  1911. 


INDEX 


Absorption,  cell  activities  and, 
129 

chemical  changes  during,  133, 
134 

definition  of,  45 

dialysis  and,  128 

effect  of   concentration   upon, 
130 
of  condiments  upon,  130 

in  small  intestine,  130-133 

of  alcohol,  129 
Adipose  tissue,  136 
Adrenal   bodies,    internal    secre- 
tion of,  242 
Alcohol,  30,  227 

absorption  of,  129 

as  cerebral  alterative,  233 

as  drug,  232 

as  food,  230 

as  poison,  236 

as  relish,  229 

historical,  227,  228 
Alimentary  canal,  54 

glycosuria,  140 
AlveoU  of  glands,  44 
Amino-acids,  "  deaminization  "  of, 
153 

from  gelatin,  149 

from  proteins,  94,  147,  148 

surplus  acids,  151 
Amylopsin  in  pancreatic  juice,  91 
Animals   as   transformers  of  en- 
ergy, 176 


Antiperistalsis  in  colon,  100 
Antrum,  70,  73 
Assimilation,  definition  of,  13 
Auricle,  116,  120 
Auto-intoxication,  206,  213 

Bile,  description  of,  95 
pigments  in,  95 
salts  in,  96 
waste  products  of,  95 
Blood  as  carrier,  105 

as    equalizer    of    temperature, 

106 
coagulation  of,  113 
corpuscles  in,  108 
plasma  of,  110 
Blood-flow,  character  of,  122 
in  arteries,  122 
in  veins,  122 

intermittency  of,  123-125 
pressure,  122,  123 
regulation  of,  249-254 
velocity  of,  125 
Body  cavity,  definition  of,  55,  56 
composition  of,  22-27 
temperature,  changes  in,  197, 
203 
of  metabolism  in  relation 
to      perspiration      and, 
198-200,  201 
humidity  and,  200,  201 
maintenance  of,  196-203 
during  exercise,  202 
265 


266 


INDEX 


Brain,  localization  of  function  in, 

258,  259 
Breathing,  162,  163 


Calorie,  definition  of,  176 
Calorimetry,  180 
direct,  183 
indirect,  184 
Capillaries,  19,  116 
Carbohydrates  in  blood,  111,  140 
in  body,  25 
in  diet,  24 
in  fasting,  171,  193 
in  liver,  138 
metabolism  of,  138-146 
Carbon    dioxid,    elimination    of, 
160-164 
in  expired  air,  163,  164 
from  protein,  carbohydrate, 

and  fat,  172 
as    stimulus    to    respiratory 
center,  164,  248 
retention,  175 
Cardia,  57 

Cardiac  sphincter,  effect  of  acid 
upon,  71 
regulation  of,  70 
Cecum,  58,  98 
Cells,  12,  15 
Central  resistance,  52 
Chocolate,  value  of,  in  diet,  225 
Cholesterin  in  bile,  95 
Chyme,  73,  75 
Circulation,  portal,  120 
pulmonary,  118 
systemic,  116 
Coagulation  of  blood,  113 
fibrin  and,  113 
thrombin  and,  113 
Coffee,  food  value  of,  224 
Colon,  58 


Colon,  absorption  in,  130-133 

antiperistalsis  in,  100 

movements  of,  100 

peristalsis  in,  88,  100,  101 
Connective  tissue,  37,  149 
Contraction,  14,  36-42 
Co-ordination,  17,  245 
Corpuscles  in  blood,  108 

red,  107 

white,  110 
Creatinin  from  protein,  156 


Deaminization,  153 

Dextrose  from  amino-acids,  154 

Diabetes,  cause  of,  145 

light  thrown  on  protein  metab- 
olism by,  154 
Dialysis  and  absorption,  128 

definition  of,  128 
Diastatic  enzymes,  32,  67,  91 
Digestion,  children  and,  205 
cooking  an  aid  in,  30 
definition  of,  28 
effect  of  fatigue  upon,  206 
of  nervous   condition  upon, 
204 
gastric,  85 
intracellular,  34 
peptic,  86 

quantity  of  food  and,  212 
salivary,  67 
tryptic,  92,  93 
Digestive  juices,  action  of,  31-35 
gastric  juice,  78-87 
intestinal,  93 
pancreatic  juice,  91-93 
saliva,  63-65,  67 
Disaccharids,  26,  94 
Diuretics,  action  of,  167 
Ductless  glands,  45,  238 
Duodenum,  57 


INDEX 


267 


Elimination   of    carbon   dioxid, 
160-164 

of  nitrogen,  165-168 
Endogenous  metabolism,  156 
Energy,  animals  as  transformers 
of,  176 

from  various  foods,  176-178 

of  metabolism,  178 

of  muscular  contraction,  37,  38 

plants  and,  20-22 

source  of,  for  work,  187-191 

transformation  of,  181-183 
Enterokinase  in  intestinal  juice, 

92 
Enzyme   action,    energy   change 
in,  35 
equilibrium,  33 
Enzymes,  action  of  acids  and  al- 
kalies upon,  34 

amylopsin,  91 

definition  of,  32 

diastatic,  32,  67,  91 

erepsin,  94,  134 

gastric  lipase,  87 

invertase,  94 

lactase,  94 

lipase  (pancreatic),  92 

lipolytic,  87,  92 

maltase,  94 

pepsin,  86 

proteolytic,  86 

ptyalin,  67 

relation  to  temperature,  34 

rennin,  85 

steapsin,  92 

thrombin,  114 

trypsin,  92 
Equilibrium  in  weight,  175 

nitrogen,  174 
Erepsin  in  intestinal  juice,  94 
Esophagus,  peristalsis  in,  66 
Exogenous  metabolism,  156 


Extractives,    drawbacks    of,     in 
diet,  220,  221 
exciters  of  gastric  secretion,  83 
in  diet,  26 

Fasting,   carbohydrate    in,   171, 
193 
fat  in,  171 
Fat,  definition  of,  25 

derived     from     carbohydrates 

and  fats,  136,  142 
distribution  of,  in  body,  25,  136 
in  diet,  26 
in  plasma,  112 
substituted  for  glycogen,  144 
''Faulhorn  experiment,"  188 
Feces  as  bearers  of  wastes,  168, 
169 
composition  of,  102 
Feeding  and  metabolism,  192 
Fever,  203 

Fibrin  in  clotting,  113 
Fletcherism,  210,  211 
Food  accessories  in  diet,  224 
action  of  bacteria  upon,  31 
purpose  of,  13,  135 
Fuel  value,  definition  of,  176 
of  carbohydrates,  177 
of  fats,  177 
of  hydrogen  and  carbon,  176, 

177 
of  proteins,  177,  178 
Fundus,  definition  of,  70 
function  of,  72 
movements  in,  72,  76 

Gastric  digestion,  85 
juice,  78-87 

acid  of,  79,  80 

Spallanzani  and,  78 
lipase,  87 


268 


INDEX 


Gastric  secretion  excited  by  dif- 
ferent means,  81-84 

Gelatin,  defective  protein,  149 
nutritive  value,  149,  150 

Glands,  ductless,  45,  238 
nervous  control  of,  42,  81 
work  of,  42,  44 

Glycogen  from  protein,  155 
from  sugar  in  blood,  138 

Glycosuria,  alimentary,  140 

Habit,  53,  258 

Heart,  anatomy  of,  116 

beat  of,  119 

nervous  control  of,  249,  250 

valves  in,  119 
Hormone,  action  of,  145,  146 

definition  of,  45 

secreted  by  pancreas,  145,  146 
Humidity  and  body  temperature, 

200,  201 
Hydrolysis.     See  Digestion. 
Hydrolytic  cleavages,  30 

Ileocecal  valve,  62,  100 
Ileum,  57 
Income,  13 
Inhibition,  62,  100 
Internal  secretion,  definition  of, 
44,  238 

of  adrenal  bodies,  242 

of  pancreas,  145 

of  reproductive  organs,  241 

of  thyroid,  239,  240 
Intestinal  juices,  93 

enterokinase  in,  92 

enzymes  in,  94 
Intestine.     See  Small  Intestine  or 

Colon. 
Invertase  in  intestinal  juice,  94 
Irreciprocal  permeability,  133 


Jejunum,  57 

Kidneys  as  organs  of  excretion, 
165-168 
blood  supply  of,  165 
work  of,  217 

Lactase  in  intestinal  juice,  94 
Large  intestine,  bacterial  action 
in,  102,  206 
effect  of  cellulose  upon,  103 
Larynx,  65 

Lipase,  definition  of,  32 
in  gastric  juice,  87 
in  pancreatic  juice,  92 
Lipolytic  enzymes,  87,  92 
Liver,  deaminization  of  proteins 
by,  153 
secretion  of  bile  by,  95 
storage  of  glycogen  in,  138 
Lymph,  composition  and  use  of, 
19 
in  vilh,  130,  131 
movement  of,  127 
Lymph-nodes,  action  of,  243 


Maltase  in  intestinal  juice,  94 
Maltose,  69 
Mastication,  63,  210 
Meat,  drawbacks  of,  as  food,  220, 
221 
in  diet,  219-222 
Mechanism,  11 
Mesentery,  60,  61 
Metabolism  and  body  tempera- 
ture, 207 
calculation  of,  169-175 
carbohydrate,  138-146 

relation  of  pancreas  to,  145 
endogenous,  156 


INDEX 


269 


Metabolism,  energy  of,  176-186 
exogenous,  156 
fats,  136-138 
feeding  and,  192 
modifications   of,   by  age  and 
sex,  194,  195 
by  mental  state,  190-192 
by  muscular  work,  187-190 
total  daily,  178 
Milk,   acid  fermentations  of,   in 
stomach,  81 
proteins  in,  150 
sour,  214 
Mineral  salts  in  diet,  225,  226 
Mouth,  54,  64 
Mucin,  65 

Muscles,     connection     of,     with 
nerves,  39,  40 
efficiency,  38 
properties,  36-42 
skeletal,  36 
Muscular  contraction,  14,  36-42 
tone,  41 


Nervous  system,  17,  18,  46-53, 

245-262 
Nitrogen,  calculation  of,  170 

elimination  of,  165-168 

from  protein,  170 

in  feces,  102 

in  urine,  166 

retention  of,  174 
Nitrogenous  equilibrium,  174 

metabolism,  147-159 
Nutrition,  bacteria  and,  157,  206, 
214 

hygiene  of,  204-226 


Outgo,  169-175 

Oxygen  carried  by  blood,  109, 162 


Pancreas,  internal  secretion  of, 
145 

relation  to  carbohydrate  met- 
abolism and  diabetes,  145 
Pancreatic  juice,  91-93 
cause  of  secretion,  91 
enzymes  in,  91-93 
Parotid  gland,  56 
Pepsin  in  gastric  secretion,  86 
Peptic  digestion,  85-87 
Peptones,  86,  93 
Peristalsis  in  esophagus,  66 

in  intestine,  88,  100,  101 
Peritoneum,  60,  61 
Permeable  membranes,  128 
Perspiration  and  body  tempera- 
ture, 198-200,  202 

as  carrier  of  waste,  168 

composition  of,  168 
Pharynx,  56 

Plants,  energy  and,  20-22 
Plasma,  blood,  110 

carbohydrates  in.  111 

composition  of,  110-113 

fats  in,  112 

proteins  in,  111 

urea  in,  112 
Portal  circulation,  120 
Protein,  23,  24 

defective,  148-150 

dextrose  from,  154 

extent  of  synthesis,  153 

in  body,  23 

in  diet,  26 

in  plasma.  111 

peculiarities  of,  212,  214 

perfect,  148-150 

place  of  synthesis,  152,  157 

synthesized   by   animals,    147, 
148 
from  amino-acids,  150-153 
Proteolytic  enzymes,  86,  92,  94 


270 


INDEX 


Protoplasm,  13 
Psychic  secretion,  81,  82 
Ptomains,  31 
Ptyalin,  67 

Pulmonary  circulation,  118 
Pyloric  sphincter,  effect  of  acid 
upon,  74-76 
regulation  of,  74-76 
Pylorus,  57 


Rations,  changes  in,  192 
effect  of  sex,  194,  195 
reduction  of,  207-209 
Receptors,  246,  260,  261 
Rectal  feedmg,  103,  104 
Rectum,  101 
Red  corpuscles,  107 

description  of,  107-109 

function  of,  109 

hemoglobin,  108 

origin,  109,  110 
Reflex  action,  46-53 
Rennin  in  gastric  secretion,  85 
Reproductive     organs,     internal 

secretion  of,  241 
Respiration,  13,  14,  160-165 
Respiratory  center,  248 
quotient,  definition  of,  172 

variations  in,  173 
Rhythmic  segmentation,  89 


Saliva,  63-65,  67 

mucin  in,  65 

ptyalin  in,  67 
Salivary    digestion    in    stomach, 
67-69 

glands,  56,  57 
Secretin,  91 
Secretions,  42-44 
Secretory  nerves,  42 


Sensations,  general,  260 

special,  260 
Shivering,  201 
Skeletal  muscles,  36 
Skin  as  organ  of  excretion,  168 
Small    intestine,    absorption    in, 
130-133 
digestion  in,  97 
peristalsis  in,  88 
rhythmic    segmentation    in, 

89 
structure,  57,  58,  62 
villi,  130 
Sour  milk,  214 
Specific  dynamic  effect  of  protein, 

193,  194 
Spleen,  function  of,  244 
Starch,  hydrolysis  of,  67-69,  91 
Steapsin     in     pancreatic     juice, 

92 
Stomach,  absorption  from,   129, 
130 
antrum  of,  70,  73 
cardia  of,  57 
fundus  of,  70-72,  76 
glands  of,  79 
movements  of,  70-77 
nervous  control  of,  74 
peristalsis  in,  73 
pylorus  of,  57 

salivary  digestion  in,  67-69 
share  of  digestion,  86,  87 
x-ray  studies  of,  72-76 
Sublingual  gland,  57 
Submaxillary  gland,  57 
Sugar    absorbed    from   stomach, 
130 
and  teeth,  223 
in  diet,  222 
in  urine,  140 
Swallowing,  65-67 
Systemic  circulation,  116 


INDEX 


271 


Tea,  food  value  of,  224 

Tendon,  37 

Thrombin  in  coagulation,  113 

Thyroid  gland,  239 
and  cretinism,  240 
and  goiter,  239 
secretion  of,  239-241 

Transformation  of  energy,   181- 
183 

Transverse  band,  70 

Trypsin,  activation  of,  92 
in  pancreatic  juice,  92 

Trypsinogen,  92 

Tryptic  digestion,  92,  93 


Urea  from  amino-acids,  154,  165 

in  plasma,  112 

in  urine,  165 
Uric  acid  in  urine,  166 

properties  of,  166,  221 
Urine,  action  of  salts  upon,  226 

albumin  in,  167 

composition  of,  165-167 

secretion  of,  by  kidneys,  165- 
168 


Urine,  sugar  in,  140 
urea  in,  165 
uric  acid  in,  166 


Ventricle,  116,  119,  120 
"Vicious  cycle,"  206,  207 
Villi,  130 

in  absorption,  130 
Vitalism,  11 
Vomiting,  76,  77 

Waste  products  in  bile,  95 
in  blood,  112 
in  feces,  102,  168,  169 
Water  as  metabolic  product,  160 
drinking,  excretion  and,  218 
meals  and,  218,  219 
nutrition  and,  217-219 
effect    of    temperature    upon 

elimination  of,  199,  200 
elimination  of,  by  kidneys,  164 
White  corpuscles,  110 

a;-RAY  studies  of  stomach,  72-76 


S'Vi  Us 


